Put money into underwear (6)
I believe the answer is:
invest
'put money' is the definition.
(I've seen this before)
'into underwear' is the wordplay.
'into' becomes 'in'.
'underwear' becomes 'vest'.
'in'+'vest'='INVEST'
(Other definitions for invest that I've seen before include "Play the market" , "Place money in a scheme" , "Put money into a company" , "Contribute money in hope of a return" , "Lay siege" .)