Graceful girl is so meaningful at heart (7)
I believe the answer is:
lissome
'graceful' is the definition.
(similar in meaning)
'girl is so meaningful at heart' is the wordplay.
'at heart' indicates a hidden word (letters hidden in the heart of the clue).
'LISSOME' can be found hidden inside 'girl is so meaningful'.
(Other definitions for lissome that I've seen before include "So smile, being agile and lithe" , "pliable" , "Slim, supple, graceful" , "Lithe; nimble" , "Thin - supple - graceful" .)