Former partner, a stud -- he could be worn out! (9)
I believe the answer is:
exhausted
'worn out' is the definition.
(exhausting is a kind of wearing out)
'former partner a stud he could be' is the wordplay.
'former partner' becomes 'ex'.
'could be' indicates an anagram.
'a'+'stud'+'he'='astudhe'
'astudhe' with letters rearranged gives 'hausted'.
'ex'+'hausted'='EXHAUSTED'
(Other definitions for exhausted that I've seen before include "Dead-beat" , "Worn out" , "Complete fatigued" , "ready for bed" , "Drained of strength" .)