Defensive structure in support of lock (8)
I believe the answer is:
fortress
'defensive structure' is the definition.
(fortress is a kind of defensive structure)
'in support of lock' is the wordplay.
'in support of' becomes 'for' (as in being for a particular cause).
'lock' becomes 'tress' (both can mean a piece of hair).
'for'+'tress'='FORTRESS'
(Other definitions for fortress that I've seen before include "Castle" , "Protective structure" , "it's hard to break into" , "Strong point" , "Place that's secure" .)