Move like a snake casting skin, being slim and flexible (5)
I believe the answer is:
lithe
'slim and flexible' is the definition.
(lithe means flexible)
'move like a snake casting skin' is the wordplay.
'move like a snake' becomes 'slither' (I've seen this before).
'casting skin' means to look at the middle letters (the 'skin' of the word is removed).
The central letters of 'slither' are 'LITHE'.
'being' acts as a link.
(Other definitions for lithe that I've seen before include "Elastic" , "showing flexibility" , "Don -, opera by Verdi" , "quick to move" , "Lissome, supple" .)