Discovers taking top off makes money (5)
I believe the answer is:
earns
'makes money' is the definition.
(I've seen this before)
'discovers taking top off' is the wordplay.
'discovers' becomes 'learns' ('learn' can be a synonym of 'discover').
'taking top off' means to remove the first letter.
'learns' with its initial letter taken away is 'EARNS'.
(Other definitions for earns that I've seen before include "Works for pay" , "Acquires by effort" , "Merits through work" , "Grosses" , "makes money" .)