Watch muscle is in condition (8)
I believe the answer is:
spectate
'watch' is the definition.
(spectating is a kind of watching)
'muscle is in condition' is the wordplay.
'muscle' becomes 'pec' (I've seen this in other clues).
'is in' means one lot of letters goes inside another.
'condition' becomes 'state' (I've seen this before).
'pec' put into 'state' is 'SPECTATE'.
(Other definitions for spectate that I've seen before include "Observe" , "Watch (eg, a game)" , "Look on (at a match)" , "Perhaps enjoy sport" , "Be an onlooker" .)