I’m in favour of exercising — right unfit (8)
I believe the answer is:
improper
'unfit' is the definition.
(I've seen this before)
'i'm in favour of exercising right' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'exercising' becomes 'pe' (abbreviation for Physical Education).
'right' becomes 'r' (common abbreviation).
'im'+'pro'+'pe'+'r'='IMPROPER'
(Other definitions for improper that I've seen before include "Not fitting" , "vulgar" , "Not decent, unseemly" , "Indecent or unsuitable" , "Not in accord with norms of behaviour" .)