Relatives installing large oven (4)
I believe the answer is:
kiln
'oven' is the definition.
(I know that kiln is a type of oven)
'relatives installing large' is the wordplay.
'relatives' becomes 'kin' (kin can mean relatives or family).
'installing' means one lot of letters goes inside another (inserted letters are installed).
'large' becomes 'L' (eg in clothes sizes).
'kin' enclosing 'l' is 'KILN'.
(Other definitions for kiln that I've seen before include "Oven for firing bricks" , "Oven for firing pottery" , "Oven for firing ceramics etc" , "Large commercial oven" , "Large oven, for potters say" .)