The Cobras believed they had earned a vital point when captain Charlie Taylor scored in the 92nd minute, but Hlabangana’s winner was the final act in what turned out to be a thrilling contest.
Southwell had taken a deserved 2-0 lead thanks to goals from Hlabangana and Callum Law before McKenzie Bult and Taylor drew the game level.
However, the goal at the death leaves Clipstone needing favourable results from elsewhere to continue their playoff pursuit with only two games left to play.
Clipstone assistant manager Ian Birtley was downbeat after the defeat but urged his side to keep fighting in the last two games. Birtley said: “It's not over, but a point or three tonight would’ve been massive.
“To lose the game in that manner is difficult. But I’ve said it all season, if you give teams a two-goal head start, that cannot happen.
“The reality is we don’t deserve to be in that top six, the table doesn’t lie. We move on, finish the season, and see where it takes us.”
Billed as a “cup final” for the hosts, Southwell was determined not to roll over and started strongly.
Clipstone goalkeeper Charlie Dando was forced into several early, albeit routine saves. But it wasn’t long before his goal was breached as Southwell took a 22nd-minute lead.
Hlabangana worked space on the edge of the box and stuck cleanly into the top left corner as Dando was left helpless.
The visitors had strong claims for a penalty waved away just before half time following a clumsy challenge. However, two minutes after the break, the referee did point to the spot.
A defensive mix-up at the back between the goalkeeper and defender brought down the Southwell striker, Law, who then stepped up and converted from 12 yards.
With a mountain to climb, Clipstone began to create their own chances, but it wasn’t until the 75th minute that they halved the deficit.
Bult, who had come on as a substitute moments before, was the first to react as the ball flashed across the face of the goal and tapped in his first goal for the club.
After sustained pressure in the closing stages, the hosts were finally rewarded when Taylor smashed home from close range following a goalmouth scramble in stoppage time.
Such is football; Clipstone had barely finished celebrating the late equaliser before Southwell inflicted one final twist in the tale.
With the game's last attack, Hlanbangana skilfully evaded three defenders and finished emphatically to take all three points back to Southwell.
Clipstones' remaining two fixtures have them hosting Stapleford Town and travelling to Rainworth, where only six points will suffice, though even that may not be enough.