Used pound found in street (5)
I believe the answer is:
spent
'used' is the definition.
(spending is a kind of using)
'pound found in street' is the wordplay.
'pound' becomes 'pen' (I've seen this before).
'found in' means one lot of letters goes inside another.
'street' becomes 'st' (abbreviation).
'pen' going into 'st' is 'SPENT'.
(Other definitions for spent that I've seen before include "Worn Out, exhausted" , "(Of resources) exhausted" , "Burned-out" , "Completely exhausted" , "Worn out - after shopping?" .)