Backing is fickle, as is posture (10)
I believe the answer is:
assistance
'backing' is the definition.
(thesaurus)
'fickle as is posture' is the wordplay.
'fickle' is an anagram indicator.
'posture' becomes 'stance' (stance is a kind of posture).
'as'+'is'='asis'
'asis' is an anagram of 'assi'.
'assi'+'stance'='ASSISTANCE'
'is' is the link.
(Other definitions for assistance that I've seen before include "Support" , "Aid" , "Help" .)