Slag off traffic - but it keeps you in gear! (3,5)
I believe the answer is:
rag trade
'it keeps you in gear' is the definition.
The definition suggests a singular noun which matches the answer.
'slag off traffic' is the wordplay.
'slag off' becomes 'rag' (I am not sure about this - if you are sure you should believe this answer much more).
'traffic' becomes 'trade' (traffic can mean illegal trade).
'rag'+'trade'='RAG TRADE'
'but' is the link.
Can you help me to learn more?
(Other definitions for rag trade that I've seen before include "newsagent's business?" , "Clothing business (colloq.)" , "Informally, the clothing business" , "Commonly, the clothing business" , "National winner" .)