Bachelor unwell, finding invoice (4)
I believe the answer is:
bill
'invoice' is the definition.
('bill' can be a synonym of 'invoice')
'bachelor unwell' is the wordplay.
'bachelor' becomes 'b' (abbreviation e.g. BA=Bachelor of Arts).
'unwell' becomes 'ill' ('ill' can be similar in meaning to 'unwell').
'b'+'ill'='BILL'
'finding' is the link.
(Other definitions for bill that I've seen before include "following 28 [SILVER]" , "beak demanded" , "Reckoning" , "Bird's beak or statement for payment" , "one settles with in America?" .)