Strike-breaker takes second taxi (4)
I believe the answer is:
scab
'strike-breaker' is the definition.
(I've seen this before)
'second taxi' is the wordplay.
'second' becomes 's' (common abbreviation).
'taxi' becomes 'cab' ('cab' can be a synonym of 'taxi').
's'+'cab'='SCAB'
'takes' acts as a link.
(Other definitions for scab that I've seen before include "Derogatory term for a strike-breaker" , "Dried crust on sore" , "Crust formed over wound" , "Strike breaker" , "cover" .)