Draw in muscles with heavenly body (6)
I believe the answer is:
absorb
'draw in' is the definition.
(I've seen this before)
'muscles with heavenly body' is the wordplay.
'muscles' becomes 'abs' (I've seen this before).
'with' means one lot of letters go next to another.
'heavenly body' becomes 'orb' (I've seen this before).
'abs'+'orb'='ABSORB'
(Other definitions for absorb that I've seen before include "Assimilate or take in as a sponge" , "Occupy the mind fully as good book will" , "Take in, water or knowledge" , "Understand" , "Soak up (liquid)" .)