That is muscle one would work on defiantly? (4)
I believe the answer is:
scab
'one would work on defiantly?' is the definition.
(worker refusing to join a strike)
'that is muscle' is the wordplay.
'that is' becomes 'sc' (abbrv. for Latin scilicet; 'namely').
'muscle' becomes 'ab' (short for abdominal muscle).
'sc'+'ab'='SCAB'
(Other definitions for scab that I've seen before include "Blackleg, strike-breaker" , "Dried surface of wound" , "Scoundrel" , "Sore - blackleg" , "cover" .)