Curtailed suggestion about pound in vain (4)
I believe the answer is:
idle
'in vain' is the definition.
(I've seen this before)
'curtailed suggestion about pound' is the wordplay.
'curtailed' means to remove the last letter.
'suggestion' becomes 'idea' (I've seen this before).
'about' is an insertion indicator.
'pound' becomes 'l'.
'idea' with its last letter taken off is 'ide'.
'ide' enclosing 'l' is 'IDLE'.
(Other definitions for idle that I've seen before include "having been taken from service" , "Useless" , "Explosive stuff" , "Unoccupied (time); run (engine) slowly" , "Ineffectual" .)