Former partner with energy cries out for physical activity (8)
I believe the answer is:
exercise
'for physical activity' is the definition.
(I've seen this before)
'former partner with energy cries out' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'with' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'out' is an anagram indicator (out can mean wrong or inaccurate).
'cries' with letters rearranged gives 'rcise'.
'e' put inside 'rcise' is 'ercise'.
'ex'+'ercise'='EXERCISE'
(Other definitions for exercise that I've seen before include "Use" , "Keep-fit" , "Employment" , "Workout" , "Task for pupil" .)