Puts his cash into underwear (7)
I believe the answer is:
invests
'puts his cash' is the definition.
'invests' can be an answer for 'puts' ('invest' can be a synonym of 'put'). I am unsure of the 'his cash' bit.
'into underwear' is the wordplay.
'into' becomes 'in'.
'underwear' becomes 'vests' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in'+'vests'='INVESTS'
Can you help me to learn more?
(Other definitions for invests that I've seen before include "Lays out, hoping for profit" , "Speculates" , "Gives significance to" , "Puts money into a venture in expectation of a return" , "Puts money into business" .)