Sensible partner once getting exercise and new addition to weight loss plan (9)
I believe the answer is:
expedient
'sensible' is the definition.
'partner once getting exercise and new addition to weight loss plan' is the wordplay.
I cannot really see how this works, but
'partner' could be 'n' and 'n' is located in the answer.
'once' could be 'ex' and 'ex' is found within the answer.
'exercise' could be 'pe' (Physical Education) and 'pe' is located in the answer.
'weight loss plan' could be 'diet' and 'diet' is found within the leftover letters.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
(Other definitions for expedient that I've seen before include "Convenient rather than right" , "Appropriate though not necessarily fair" , "Opportune" , "Convenient for a particular purpose" , "Means to get" .)