What doors should do is obvious (4-3-4)
I believe the answer is:
open-and-shut
I believe this clue is a double definition.
'what doors should do' is the first definition.
(I've seen this before)
'obvious' is the second definition.
(I've seen this before)
'is' is the link.
(Other definitions for open-and-shut that I've seen before include "Straightforward" , "So obvious as to be easily decided" , "No nude paths (anag) - easily decided" .)