What most shops do most days is obvious (4-3-4)
I believe the answer is:
open-and-shut
'obvious' is the definition.
(I've seen this before)
'what most shops do most days' is the wordplay.
I cannot really see how this works, but
'do' could be 'ut' (synonyms) and 'ut' is found in the answer.
'days' could be 'd' (abbreviation for days) and 'd' is found within the answer.
This explanation may well be incorrect...
'is' acts as a 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" .)