Reinforce only the lock (8)

I believe the answer is:

buttress

'reinforce' 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'

(Other definitions for buttress that I've seen before include "that goes flying round the cathedral?" , "She-goat" , "It may be flying" , "Strengthen" , "Stone support" .)

I've seen this clue in the Evening Standard.

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