Sentimental student left - careless (5)
I believe the answer is:
soppy
'sentimental' is the definition.
(I've seen this in another clue)
'student left careless' is the wordplay.
'student' becomes 'L' (as in L-plates for learner drivers).
'left' is a deletion indicator.
'careless' becomes 'sloppy' (synonyms).
'sloppy' with 'l' taken away is 'SOPPY'.
(Other definitions for soppy that I've seen before include "Indulgently sentimental" , "Overemotional" , "Emotional" , "Feeble; sentimental" , "Wet; sentimental" .)