Firm writes to stop colleague's refund (10)
I believe the answer is:
compensate
'refund' is the definition.
(thesaurus)
'firm writes to stop colleague's' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'writes' becomes 'pens' ('pen' can be a synonym of 'write').
'to stop' indicates putting letters inside (stop can mean to plug a hole in something).
'colleague' becomes 'mate' (mate can mean a colleague or friend).
'pens' placed within 'mate' is 'mpensate'.
'co'+'mpensate'='COMPENSATE'
(Other definitions for compensate that I've seen before include "Reimburse" , "attempt to limit damage" , "Make good" , "Make financial amends to" , "Balance" .)