Thought money off did apply (4)
I believe the answer is:
used
'did apply' is the definition.
('use' can be a synonym of 'apply')
'thought money off' is the wordplay.
'thought' becomes 'mused' (musing is a kind of thinking).
'money' becomes 'm' (abbreviation in economics).
'off' is a deletion indicator.
'mused' with 'm' taken out is 'USED'.
(Other definitions for used that I've seen before include "Second-hand; consumed" , "Brought into play" , "Pre-owned" , "was milking" , "Previously owned by another" .)