Dog second in queue (8)
I believe the answer is:
'queue' is the definition.
(I have seen 'Queue of cars ' mean 'tailback' so perhaps 'queue' could also mean 'tailback')
'dog second' is the wordplay.
'dog' becomes 'tail' (synonyms).
'second' becomes 'back' (to second a motion is to back it).
'in' acts as a link.
(Other definitions for tailback that I've seen before include "Queue of cars" , "Traffic hold-up" , "Long queue of traffic" , "Traffic jam" , "Lack a bit (anag.)" .)