Pupil is a bit pliant (7)
I believe the answer is:
lissome
'pliant' is the definition.
(thesaurus)
'pupil is a bit' is the wordplay.
'pupil' becomes 'l'.
'a bit' becomes 'some' (I've seen this in other clues).
'l'+'is'+'some'='LISSOME'
(Other definitions for lissome that I've seen before include "pliable" , "So smile, being agile and lithe" , "Slim, supple, graceful" , "flexible" , "Lithe; nimble" .)