Clark was at the John Smith's Stadium for the first time since being sacked 11 months ago and had looked set to pile greater pressure on successor Simon Grayson.
But substitute Hammill popped up with almost the last kick of the game to level in what is scheduled to be the last match of his loan spell before returning to Wolves.
Birmingham's Academy graduate Callum Reilly looked to have settled matters from 10 yards with his first goal for the club on the stroke of half-time.
But Clark's heartbroken young guns can take plenty of credit despite now boasting just two wins in 12 league games, with Grayson having plenty more to worry about after a lacklustre display which rescued a last-gasp point but keeps Town winless in 11.
The first opening arrived midway through the half but Jack Butland was equal to Sean Scannell's acrobatic kick from Neil Danns' run and cross.
Blues finally threatened in the 41st minute but Curtis Davies' overhead kick went wide.
The lively Chris Burke tested Alex Smithies before then firing across the face of goal as the visitors upped the tempo before half-time.
And they were rewarded for their pressure in first-half injury time as Burke's cross was headed back into the path of Reilly who smashed home from 10 yards.
The teenager's afternoon, however, came to an end at the interval as he was replaced by Jonathan Spector, presumably due to injury.
Huddersfield were lacking any impetus at the start of the second half and it was Birmingham who appeared in control, Danns fired over the bar from 12 yards but Town were creating very little, with Grayson a frustrated figure on the sidelines.
Smithies parried well from Nathan Redmond in the 77th minute as Blues threatened a second goal which would have ended the contest.
Jermaine Beckford headed wide for the Terriers, who were twice thwarted by Butland as the game approached injury time.
Substitute Alan Lee was first to be denied as his close-range effort was parried away, with Beckford seeing his header well kept out from the subsequent corner.
But Hammill rescued a point for the hosts in the 93rd minute when he beat Butland from 12 yards.