Reinforce lock on bar (8)
I believe the answer is:
buttress
'reinforce' is the definition.
(thesaurus)
'lock on bar' is the wordplay.
'lock' becomes 'tress' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'bar' becomes 'but' (I've seen this in other clues**).
'tress' put after 'but' is 'BUTTRESS'.
(Other definitions for buttress that I've seen before include "It may be flying" , "Reinforce" , "support for Notre Dame?" , "strengthening side" , "that goes flying round the cathedral?" .)