Second taxi is for a strike-breaker (4)
I believe the answer is:
scab
'for a strike-breaker' is the definition.
(I've seen this in another clue)
'second taxi' is the wordplay.
'second' becomes 's' (common abbreviation).
'taxi' becomes 'cab' (synonyms).
's'+'cab'='SCAB'
'is' is the link.
(Other definitions for scab that I've seen before include "Strike-breaker; skin disease in animals" , "Wound's crust" , "Blackleg, strike-breaker" , "Crust formed over wound" , "Disease of sheep" .)