Copper's caught by top defender's first defendant (7)
I believe the answer is:
accused
'defendant' is the definition.
('accused' can be a synonym of 'defendant')
'copper's caught by top defender's first' is the wordplay.
'copper's' becomes 'cus' (chemical symbol).
'caught by' is an insertion indicator.
'top' becomes 'ace' (**).
'first' suggests taking the first letters.
The initial letter of 'defenders' is 'd'.
'cus' put inside 'ace' is 'accuse'.
'accuse'+'d'='ACCUSED'
(Other definitions for accused that I've seen before include "Charged with an offence" , "The person in the dock" , "Indicted" , "Incriminated" , "Levelled a charge against" .)