Second taxi for blackleg (4)
I believe the answer is:
scab
'for blackleg' is the definition.
(synonyms)
'second taxi' is the wordplay.
'second' becomes 's' (common abbreviation).
'taxi' becomes 'cab' (synonyms).
's'+'cab'='SCAB'
(Other definitions for scab that I've seen before include "Dried surface of wound" , "Evidence of healing wound - crosser of picket line" , "Non-striker" , "work when others are out" , "complaint with scales" .)