Suitable former exercise regime any number accepted (9)
I believe the answer is:
expedient
'suitable' is the definition.
(I've seen this before)
'former exercise regime any number accepted' is the wordplay.
'former' becomes 'ex'.
'exercise' becomes 'pe' (Physical Education).
'regime' becomes 'diet' (I've seen this before).
'any number' becomes 'n' (I've seen this in other clues).
'accepted' indicates putting letters inside.
'diet' enclosing 'n' is 'dient'.
'ex'+'pe'+'dient'='EXPEDIENT'
(Other definitions for expedient that I've seen before include "Suitable - practical - device" , "Practical - advantageous" , "Serving to promote one's interest" , "Advantageous in a given situation" , "Suitable for the circumstances rather than just" .)