Fitness system good, least bad (7)
I believe the answer is:
pilates
'fitness system' is the definition.
(I've seen this in another clue)
'good least bad' is the wordplay.
'good' becomes 'pi' (old-fashioned term for pious).
'bad' is an anagram indicator.
'least' with letters rearranged gives 'lates'.
'pi'+'lates'='PILATES'
(Other definitions for pilates that I've seen before include "Pleats I arrange into a form of exercise" , "Set of exercises used in holistic medicine" , "Modern exercise" , "Holistic system of physical exercises" , "discipline?" .)