Leaves behind old-fashioned sports kits (9)
I believe the answer is:
outstrips
'leaves behind' is the definition.
(outstripping is a kind of leaving behind)
'old-fashioned sports kits' is the wordplay.
'old-fashioned' becomes 'out' (as opposed to 'in').
'sports kits' becomes 'strips' (I can't justify this - if you can you should believe this answer much more).
'out'+'strips'='OUTSTRIPS'
Can you help me to learn more?
(Other definitions for outstrips that I've seen before include "trumps" , "Overtakes; exceeds" .)