Belly laugh coming from muscular oarsman (4)
I believe the answer is:
roar
'belly laugh' is the definition.
(roaring laughter)
'coming from muscular oarsman' is the wordplay.
'coming from' indicates a hidden word.
'ROAR' is hidden in the letters of 'muscular oarsman'.
(Other definitions for roar that I've seen before include "Lion's call" , "Deep loud sound as of lion or bull" , "Loud cry" , "Great cry" , "Jaguar, for example, emits this" .)