Girl is someone keeping supple (7)
I believe the answer is:
lissome
'supple' is the definition.
('lissome' can be a synonym of 'supple')
'girl is someone keeping' is the wordplay.
'keeping' says the answer is hidden in the clue.
'LISSOME' is hidden in the letters of 'girl is someone'.
(Other definitions for lissome that I've seen before include "pliable" , "Oil mess (anag)" , "Willowy" , "Slim, supple, graceful" , "Lithe; nimble" .)