Shy student bitten by rat (5)
I believe the answer is:
sling
'shy' is the definition.
(both can mean to throw)
'student bitten by rat' is the wordplay.
'student' becomes 'L' (as in L-plates for learner drivers).
'bitten by' is an insertion indicator (I've seen this in other clues).
'rat' becomes 'sing' (both can mean to inform on someone).
'l' inserted into 'sing' is 'SLING'.
(Other definitions for sling that I've seen before include "Injured-arm support" , "Stone-throwing weapon" , "Throw, fling" , "Move off!" , "shy" .)