Spend a penny on a ship (4)
I believe the answer is:
pass
'spend' is the definition.
('pass' can be a synonym of 'spend')
'a penny on a ship' is the wordplay.
'a penny' becomes 'p'.
'on' says to put letters next to each other.
'ship' becomes 'ss'.
'p'+'a'+'ss'='PASS'
(Other definitions for pass that I've seen before include "Approve; mountain route" , "Overtake; permit" , "beats me" , "Bad situation" , "Go by; permit" .)