Wild revel in indoor gymnastics (4)
I believe the answer is:
orgy
'wild revel' is the definition.
(I've seen this before)
'in indoor gymnastics' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ORGY' can be found hidden inside 'indoor gymnastics'.
(Other definitions for orgy that I've seen before include "Unrestrained revel" , "Excess" , "Debauched party" , "Frenzy; bacchanalia" , "Wild activity" .)