In favour of lock in defensive structure (8)
I believe the answer is:
fortress
'defensive structure' is the definition.
(fortress is a kind of defensive structure)
'in favour of lock' is the wordplay.
'in favour of' becomes 'for' (I've seen this before).
'lock' becomes 'tress' (I've seen this before).
'for'+'tress'='FORTRESS'
'in' is the link.
(Other definitions for fortress that I've seen before include "Castle" , "Military stronghold" , "Secure building" , "it's hard to break into" , "Perhaps keep" .)