Acquit former partner on single charge (9)
I believe the answer is:
exonerate
'acquit' is the definition.
('exonerate' can be a synonym of 'acquit')
'former partner on single charge' is the wordplay.
'former partner' becomes 'ex'.
'on' means one lot of letters go next to another.
'single' becomes 'one' (I've seen this before).
'charge' becomes 'rate' (rate is a kind of charge**).
'ex'+'one'+'rate'='EXONERATE'
(Other definitions for exonerate that I've seen before include "let off the hook" , "Free of duty" , "Discharge" , "Release" , "Clear of blame" .)