Pound the little devil for being slack! (4)
I believe the answer is:
limp
'being slack' is the definition.
(I've seen this before)
'pound the little devil' is the wordplay.
'pound' becomes 'l' (the pound sign is based on an L).
'the little devil' becomes 'imp' (an imp is a little demon).
'l'+'imp'='LIMP'
'for' acts as a link.
(Other definitions for limp that I've seen before include "Walk stiffly - lacking energy" , "without energy" , "Walk with an uneven stop" , "Walk unevenly; not stiff" , "Lacking stiffness" .)