Material in the fireplace (3)
I believe the answer is:
rep
'material' is the definition.
(I know that rep is a type of protein)
'in the fireplace' is the wordplay.
'in' indicates the answer is hidden within the clue.
'REP' can be found hidden inside 'fireplace'.
(Other definitions for rep that I've seen before include "what a draper usually sells" , "Fabric with a ribbed surface" , "Salesperson" , "Stuff" , "Corded fabric" .)