Lock supports barring support (8)
I believe the answer is:
buttress
'support' is the definition.
(I know that buttress is a type of support)
'lock supports barring' is the wordplay.
'lock' becomes 'tress' (both can mean a piece of hair).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'barring' becomes 'but' (both can mean 'except').
'tress' put after 'but' is 'BUTTRESS'.
(Other definitions for buttress that I've seen before include "Structure providing support" , "support for Notre Dame?" , "Reinforce" , "Projecting support for building" , "Supporting structure" .)