Tell tales about revolting trainer (7)
I believe the answer is:
sneaker
'trainer' is the definition.
(both can mean a shoe worn for sports)
'tell tales about revolting' is the wordplay.
'tell tales' becomes 'sneak' (to sneak on someone is to tell tales about them).
'about' becomes 're' (regarding).
'revolting' is a reversal indicator.
're' reversed gives 'er'.
'sneak'+'er'='SNEAKER'
(Other definitions for sneaker that I've seen before include "Canvas shoe" , "one tending to go unnoticed?" , "Casual shoe" , "Soft shoe" , "Trainer (shoe)" .)