Hit the mug holding the money (4)
I believe the answer is:
slap
'hit' is the definition.
(both can mean to strike)
'mug holding the money' is the wordplay.
'mug' becomes 'sap'.
'holding' is an insertion indicator.
'the money' becomes 'l' (L can mean 'pounds').
'sap' placed around 'l' is 'SLAP'.
'the' acts as a link.
(Other definitions for slap that I've seen before include "Blow, maybe in the face" , "Strike with the palm" , "Hit with a flat hand" , "Blow with open hand" , "Strike (in the face)" .)