Set up bath and lock support (8)
I believe the answer is:
buttress
'support' is the definition.
(buttress is a kind of support)
'set up bath and lock' is the wordplay.
'set up' says the letters should be written in reverse.
'bath' becomes 'tub' ('tub' can be a synonym of 'bath').
'and' is a charade indicator (letters next to each other).
'lock' becomes 'tress' (I've seen this before).
'tub' in reverse letter order is 'but'.
'but'+'tress'='BUTTRESS'
(Other definitions for buttress that I've seen before include "Wall supporter" , "Supporting structure" , "that goes flying round the cathedral?" , "strengthening side" , "support for Notre Dame?" .)