Commercial vehicle with identification at rear of offside edge (9)
I believe the answer is:
advantage
'edge' is the definition.
(I've seen this before)
'commercial vehicle with identification at rear of offside' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'vehicle' becomes 'van' (van is a kind of vehicle).
'with' is a charade indicator (letters next to each other).
'identification at' becomes 'tag' (I can't justify this - if you can you should give a lot more credence to this answer).
'rear of' indicates one should take the final letters.
The last letter of 'offside' is 'e'.
'ad'+'van'+'tag'+'e'='ADVANTAGE'
Can you help me to learn more?
(Other definitions for advantage that I've seen before include "First point scored after deuce in tennis" , "Use" , "Edge, head start" , "Profit, tennis score" , "Superior or more favourable position" .)