Blackleg in second taxi (4)
I believe the answer is:
scab
'blackleg' is the definition.
('scab' can be a synonym of 'blackleg')
'second taxi' is the wordplay.
'second' becomes 's' (common abbreviation).
'taxi' becomes 'cab' ('cab' can be a synonym of 'taxi').
's'+'cab'='SCAB'
'in' acts as a link.
(Other definitions for scab that I've seen before include "Crusted sore on cut or wound" , "Crust of congealed blood" , "work when others are out" , "Dried crust on sore" , "Crust formed over wound" .)