Reinforcement for only the lock (8)
I believe the answer is:
buttress
'reinforcement' is the definition.
(thesaurus)
'only the lock' is the wordplay.
'only' becomes 'but' (I've seen this before).
'the lock' becomes 'tress' (I've seen this before).
'but'+'tress'='BUTTRESS'
'for' is the link.
(Other definitions for buttress that I've seen before include "Projecting support for building" , "(Architectural) support" , "It strengthens the wall" , "strengthening side" , "Strengthen" .)