Small, more supple snake (7)
I believe the answer is:
slither
'snake' is the definition.
(thesaurus)
'small more supple' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'more supple' becomes 'lither' ('lithe' can be a synonym of 'supple').
's'+'lither'='SLITHER'
(Other definitions for slither that I've seen before include "Slide (like a snake)" , "Slide on slippery surface" , "Snake along" , "Slide unsteadily" , "Move like a snake" .)