Let off former partner with single charge (9)
I believe the answer is:
exonerate
'let off' is the definition.
(thesaurus)
'former partner with single charge' is the wordplay.
'former partner' becomes 'ex'.
'with' 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 "Officially absolve from blame" , "Excuse" , "Free of duty" , "Justify" , "Acquit" .)