Old French coin originally spent on underwear (3)
I believe the answer is:
sou
'old french coin' is the definition.
(I've seen this before)
'originally spent on underwear' is the wordplay.
'originally' suggests taking the first letters.
The initial letters of 'spent on underwear' is 'SOU'.
(Other definitions for sou that I've seen before include "16 French coin" , "Tiny amount of money" , "Old French small coin" , "Old Fr. coin; sounds like entreat" , "bit of money" .)