Prop saving lock (8)
I believe the answer is:
buttress
'prop' is the definition.
(I've seen this before)
'saving lock' is the wordplay.
'saving' becomes 'but' ('but' can mean 'saving' or 'except').
'lock' becomes 'tress' (both can mean a piece of hair).
'but'+'tress'='BUTTRESS'
(Other definitions for buttress that I've seen before include "Supporting structure" , "Support built against a wall, prop" , "Strengthen" , "Prop up, brace" , "Projecting support for a 21 ac" .)