I'd sent a replacement as a substitute (7)
I believe the answer is:
instead
'as a substitute' is the definition.
(I've seen this in another clue)
'i'd sent a replacement' is the wordplay.
'replacement' is an anagram indicator.
'id'+'sent'+'a'='idsenta'
'idsenta' is an anagram of 'INSTEAD'.
(Other definitions for instead that I've seen before include "In lieu" , "for a change" , "as an alternative option" , "As a substitute" , "Rather - detains (anag)" .)