Pound belonging to Heath Robinson (5)
I believe the answer is:
throb
'pound' is the definition.
(throbbing is a kind of pounding)
'belonging to heath robinson' is the wordplay.
'belonging to' indicates the answer is hidden within the clue (I've seen this in other clues).
'THROB' is hidden in the letters of 'heath robinson'.
(Other definitions for throb that I've seen before include "Broth (anag.)" , "Drum" , "Pound" , "Martin -; Kingsley -" , "Beat rhythmically" .)