Job upset saint in one of many shops (5,5)
I believe the answer is:
chain store
'one of many shops' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'job upset saint in' is the wordplay.
'job' becomes 'chore' (synonyms).
'upset' indicates an anagram.
'in' is an insertion indicator.
'saint' is an anagram of 'ainst'.
'chore' placed around 'ainst' is 'CHAIN-STORE'.
Can you help me to learn more?
(Other definitions for chain store that I've seen before include "shop" , "One of a series of firm's shops" , "an outlet" .)