Feeling regret about student's decision (6)
I believe the answer is:
ruling
'decision' is the definition.
(I've seen this before)
'feeling regret about student's' is the wordplay.
'feeling regret' becomes 'ruing' (I've seen this in another clue).
'about' is an insertion indicator.
'student' becomes 'l' (as in L-plates for learner drivers).
'ruing' going around 'l' is 'RULING'.
(Other definitions for ruling that I've seen before include "Verdict" , "Decree" , "Judge's decision" , "Court decision" , "Judgement" .)