Acquit former copper with large head (9)
I believe the answer is:
exculpate
'acquit' is the definition.
('exculpate' can be a synonym of 'acquit')
'former copper with large head' is the wordplay.
'former' becomes 'ex'.
'copper' becomes 'cu' (chemical symbol).
'with' means one lot of letters go next to another.
'large' becomes 'l'.
'head' becomes 'pate' (I've seen this before**).
'ex'+'cu'+'l'+'pate'='EXCULPATE'
(Other definitions for exculpate that I've seen before include "Free or clear from blame" , "Acquit - clear" , "Acquit, exonerate" , "Declare someone not guilty of wrongdoing" , "Absolve (from a crime)" .)