Part of door fitted into arch, in general (5)
I believe the answer is:
hinge
'part of door' is the definition.
'fitted into arch in general' is the wordplay.
'fitted into' indicates a hidden word.
'HINGE' is hidden in the letters of 'arch in general'.
(Other definitions for hinge that I've seen before include "Pivotal point" , "Key principle" , "Part of a door" , "What a door depends upon!" , "Fastening for door etc" .)