Expenditure on bill for buttonhole (6)
I believe the answer is:
accost
'buttonhole' is the definition.
(thesaurus)
'expenditure on bill' is the wordplay.
'expenditure' becomes 'cost' (cost is a kind of expenditure).
'on' means one lot of letters go next to another.
'bill' becomes 'ac'.
'cost' put after 'ac' is 'ACCOST'.
'for' is the link.
(Other definitions for accost that I've seen before include "Come boldly up to" , "Challenge" , "Meet and greet" , "Approach and address (someone) aggressively" , "Approach boldly or impudently" .)