Merciless, cold student conceals regret (5)
I believe the answer is:
cruel
'merciless' is the definition.
(I've seen this before)
'cold student conceals regret' is the wordplay.
'cold' becomes 'c' (eg on taps).
'student' becomes 'l' (as in L-plates for learner drivers).
'conceals' is an insertion indicator.
'regret' becomes 'rue' (to rue something is to regret it).
'c'+'l'='cl'
'cl' going around 'rue' is 'CRUEL'.
(Other definitions for cruel that I've seen before include "Devoid of pity" , "Heartless" , "Inflicting pain without pity" , "Savage; harsh" , "Vicious" .)