Girl is so mesmerising keeping supple (7)
I believe the answer is:
lissome
'supple' is the definition.
('lissome' can be a synonym of 'supple')
'girl is so mesmerising keeping' is the wordplay.
'keeping' says the answer is hidden in the clue.
'LISSOME' is hidden within 'girl is so mesmerising'.
(Other definitions for lissome that I've seen before include "flexible" , "Thin - supple - graceful" , "Willowy" , "pliable" , "Oil mess (anag)" .)