In rear limb's large scab (8)
I believe the answer is:
blackleg
'scab' is the definition.
(synonyms)
'in rear limb's large' is the wordplay.
'in' means one lot of letters goes inside another.
'rear' becomes 'back' (synonyms).
'limb' becomes 'leg' (leg is a kind of limb).
'large' becomes 'L' (eg in clothes sizes).
'back'+'leg'='backleg'
'backleg' placed around 'l' is 'BLACKLEG'.
(Other definitions for blackleg that I've seen before include "Strikebreaker - disease of farm animals" , "one not out, controversially" , "Person who works while others are on strike, having dark limb" , "Scab, strikebreaker" , "One who breaks a strike by working" .)