Defence's favourite case for a charge (7)
I believe the answer is:
parapet
'defence's' is the definition.
(parapet is a kind of defence)
'favourite case for a charge' is the wordplay.
'favourite' becomes 'pet' (synonyms).
'case for' indicates putting letters inside.
'charge' becomes 'rap' (both can mean blame or responsibility).
'a'+'rap'='arap'
'pet' going around 'arap' is 'PARAPET'.
(Other definitions for parapet that I've seen before include "keep overlooking this?" , "form of defence" , "Protective cover" , "place of safety?" , "form of protection" .)