Offensive weapon student held in game (9)
I believe the answer is:
charmless
'offensive' is the definition.
(charmless can mean offensive or unpleasant)
'weapon student held in game' is the wordplay.
'weapon' becomes 'arm' (synonyms).
'student' becomes 'l' (as in L-plates for learner drivers).
'held in' is an insertion indicator.
'game' becomes 'chess' (chess game is a kind of game).
'arm'+'l'='arml'
'arml' placed into 'chess' is 'CHARMLESS'.
(Other definitions for charmless that I've seen before include "not winning" , "Unattractive" , "Without attractive features" , "Lacking any appeal" .)