Gymnastics only seen outside in public (6)
I believe the answer is:
openly
'in public' is the definition.
(I've seen this in another clue)
'gymnastics only seen outside' is the wordplay.
'gymnastics' becomes 'pe' (Physical Education).
'seen outside' is an insertion indicator.
'pe' placed inside 'only' is 'OPENLY'.
(Other definitions for openly that I've seen before include "In public" , "(Done) without concealment" , "Frankly, honestly" , "In an overt way" , "Above board" .)