Merely taking exercise in full view (6)
I believe the answer is:
openly
'in full view' is the definition.
(I've seen this before)
'merely taking exercise' is the wordplay.
'merely' becomes 'only' (synonyms).
'taking' means one lot of letters goes inside another.
'exercise' becomes 'pe' (Physical Education).
'only' enclosing 'pe' is 'OPENLY'.
(Other definitions for openly that I've seen before include "In frank and candid manner" , "without secrecy" , "Overtly" , "Candidly, without concealment." , "Above board" .)