Refund engineers with brie - sum varies (9)
I believe the answer is:
reimburse
'refund' is the definition.
(I know that reimburse is a more specific form of the action refund)
'engineers with brie sum varies' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'with' indicates putting letters inside.
'varies' is an anagram indicator.
'brie'+'sum'='briesum'
'briesum' is an anagram of 'eimburs'.
're' going around 'eimburs' is 'REIMBURSE'.
(Other definitions for reimburse that I've seen before include "Pay back (e.g. expenses)" , "Financially compensate" , "refund" , "Repay (a sum of money)" , "Compensate for money already spent" .)