Just about exercises in public (6)
I believe the answer is:
openly
'in public' is the definition.
(I've seen this before)
'just about exercises' is the wordplay.
'just' becomes 'only' ('only' can be a synonym of 'just').
'about' means one lot of letters goes inside another.
'exercises' becomes 'pe'.
'only' placed around 'pe' is 'OPENLY'.
(Other definitions for openly that I've seen before include "In public" , "In an overt way" , "without secrecy" , "Without concealment or prevarication" , "In candid, up-front manner" .)