Cost, old penny, seen to change (7)
I believe the answer is:
expense
'cost' is the definition.
(thesaurus)
'old penny seen to change' is the wordplay.
'old' becomes 'ex'.
'penny' becomes 'p'.
'to change' indicates anagramming the letters.
'seen' anagrammed gives 'ense'.
'ex'+'p'+'ense'='EXPENSE'
(Other definitions for expense that I've seen before include "What something costs" , "Cost - price" , "Disbursement" , "Outlay or cost" , "Incurred cost" .)