Grasping girl, initially tall and thin (6)
I believe the answer is:
greedy
'grasping' is the definition.
(I know that grasping can be written as greedy)
'girl initially tall and thin' is the wordplay.
'initially' suggests taking the first letters.
'tall and thin' becomes 'reedy' (I've seen this before).
The initial letter of 'girl' is 'g'.
'g'+'reedy'='GREEDY'
(Other definitions for greedy that I've seen before include "Avaricious and acquisitive" , "Grasping" , "Having a voracious appetite" , "Starving" , "Gluttonous" .)