Prop up, only lock (8)
I believe the answer is:
buttress
'prop up' is the definition.
(thesaurus)
'only lock' is the wordplay.
'only' becomes 'but' (I've seen this before).
'lock' becomes 'tress' (I've seen this before).
'but'+'tress'='BUTTRESS'
(Other definitions for buttress that I've seen before include "Projecting support for building" , "Pier" , "Support built against a wall, prop" , "It may be flying" , "Projecting support for wall" .)