Lock by only pier (8)
I believe the answer is:
buttress
'pier' is the definition.
(I've seen this before)
'lock by only' is the wordplay.
'lock' becomes 'tress' (both can mean a piece of hair).
'by' says to put letters next to each other.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'tress' put after 'but' is 'BUTTRESS'.
(Other definitions for buttress that I've seen before include "strengthening side" , "Prop up, brace" , "Projecting support for a 21 ac" , "Support built against a wall, prop" , "It strengthens the wall" .)