Call former partner over demand (7)
I believe the answer is:
exclaim
'call' is the definition.
(I've seen this before)
'former partner over demand' is the wordplay.
'former partner' becomes 'ex'.
'over' means one lot of letters go next to another.
'demand' becomes 'claim' (claiming is a kind of demanding**).
'ex'+'claim'='EXCLAIM'
(Other definitions for exclaim that I've seen before include "speak out boldly" , "Shout out" , "say!" , "Express surprise" , "Utter boldly" .)