Remember units must be changed for refund (13)
I believe the answer is:
reimbursement
'refund' is the definition.
(thesaurus)
'remember units must be changed' is the wordplay.
'must be changed' indicates anagramming the letters.
'remember'+'units'='rememberunits'
'rememberunits' is an anagram of 'REIMBURSEMENT'.
'for' acts as a link.
(Other definitions for reimbursement that I've seen before include "Repayment (of money)" , "Money to cover expenses already incurred" , "Compensation for money spent" , "Heel; dagger" .)