Only hair for support (8)
I believe the answer is:
buttress
'for support' is the definition.
(buttress is a kind of support)
'only hair' is the wordplay.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'hair' becomes 'tress' (tress is a kind of hair).
'but'+'tress'='BUTTRESS'
(Other definitions for buttress that I've seen before include "support for Notre Dame?" , "Projecting support for a 21 ac" , "Stanchion" , "Pier" , "Wall supporter" .)