Review the original item:
Fighting the urge to crawl under her desk, Adriana looked at the
clock,
a half hour of class remained, more than enough time to do her poorly
prepared, ten-minute speech.
You wanted to fix it this way:
Fighting the urge to crawl under her desk, Adriana looked at the
clock a half hour of class remained, more than
enough time to do her poorly prepared, ten-minute speech.
To delete the comma between clock and a would cause an equally bad problem, a fused sentence. A fused sentence occurs when you have two main clauses joined with no punctuation whatsoever. Adriana looked at the clock is the first main clause. A half hour of class remained is the second main clause.
The spot between clock and a needs a stronger break than blank space.
You might want to consult the rules for fixing comma splices and fused sentences.