So slim, mobile and supple (6)
I believe the answer is:
lissom
'supple' is the definition.
('lissom' can be a synonym of 'supple')
'so slim mobile' is the wordplay.
'mobile' is an anagram indicator.
'so'+'slim'='soslim'
'soslim' with letters rearranged gives 'LISSOM'.
'and' is the link.
(Other definitions for lissom that I've seen before include "Lithe, nimble" , "Limber" , "Supple, nimble" , "Lithe and flexible" , "'Lithe, supple (6)'" .)