He decides to make a rarebit (7)
I believe the answer is:
arbiter
'he decides' is the definition.
(I've seen this before)
'make a rarebit' is the wordplay.
'make' is an anagram indicator.
'rarebit' is an anagram of 'ARBITER'.
'to' acts as a link.
(Other definitions for arbiter that I've seen before include "Decision maker" , "One has the authority to settle disputes" , "Someone with the power to settle the matter" , "Judge, adjudicator" , "Rear bit (anag.)" .)