With key, locks a piece of furniture (5)
I believe the answer is:
chair
'a piece of furniture' is the definition.
(chair is a kind of piece of furniture)
'with key locks' is the wordplay.
'with' is a charade indicator (letters next to each other).
'key' becomes 'c' (I've seen this in other clues).
'locks' becomes 'hair' (I've seen this before).
'c' next to 'hair' is 'CHAIR'.
(Other definitions for chair that I've seen before include "Most important seat in committee" , "Movable seat" , "See 4" , "Head of a university faculty or department" , "Prof's post; meeting leader" .)