Part of girl is somewhat flexible (6)
I believe the answer is:
lissom
'flexible' is the definition.
(I've seen this before)
'part of girl is somewhat' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'LISSOM' can be found hidden inside 'girl is somewhat'.
(Other definitions for lissom that I've seen before include "Lithe and flexible" , "Lithe, nimble" , "'Lithe, supple (6)'" , "Supple or agile" , "Limber" .)