Proxy – but it set us up (10)
I believe the answer is:
substitute
'proxy' is the definition.
(thesaurus)
'but it set us up' is the wordplay.
'up' indicates anagramming the letters (letters get chucked up).
'but'+'it'+'set'+'us'='butitsetus'
'butitsetus' anagrammed gives 'SUBSTITUTE'.
(Other definitions for substitute that I've seen before include "Replacement" , "Put in place of another" , "Reserve" , "Stand in" , "Stand-in" .)