Second taxi for strikebreaker (4)
I believe the answer is:
scab
'for strikebreaker' 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 "that blocks cut" , "Sign of skin wound" , "Dried crust on wound" , "He's not out" , "he refuses to come out" .)