A piece put in, one to measure and harden (5)
I believe the answer is:
inset
'a piece put in' is the definition.
(I've seen this before)
'one to measure and harden' is the wordplay.
'one to measure' becomes 'in' (in is a kind of measure).
'and' says to put letters next to each other.
'harden' becomes 'set' (I've seen this in another clue).
'in'+'set'='INSET'
(Other definitions for inset that I've seen before include "lesser feature" , "Put (one thing) into (another)" , "Implant" , "Extra bit of garment" , "Stein (anag)" .)