Sent me to fill the breach, creating animosity (10)
I believe the answer is:
resentment
'animosity' is the definition.
(thesaurus)
'sent me to fill the breach' is the wordplay.
'to fill' indicates putting letters inside.
'the breach' becomes 'rent' (I've seen this before).
'sent'+'me'='sentme'
'sentme' inserted inside 'rent' is 'RESENTMENT'.
'creating' is the link.
(Other definitions for resentment that I've seen before include "Bitterness, indignation" , "ill-feeling" , "Hard feelings" , "Ill will" , "Umbrage" .)