Exercise just about in public (6)
I believe the answer is:
openly
'in public' is the definition.
(I've seen this in another clue)
'exercise just about' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'just' becomes 'only' (synonyms).
'about' means one lot of letters goes inside another.
'pe' going within 'only' is 'OPENLY'.
(Other definitions for openly that I've seen before include "In frank and candid manner" , "Frankly, honestly" , "Without concealment, frankly" , "In public" , "In an overt way" .)