Fat bags for every brand that’s a big seller (11)
I believe the answer is:
supermarket
'a big seller' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'fat bags for every brand' is the wordplay.
'fat' becomes 'suet' (suet is a kind of fat).
'bags' is an insertion indicator (bag can mean secures or steals).
'for every' becomes 'per' ('per' means 'for each' or 'for every').
'brand' becomes 'mark' (synonyms).
'per'+'mark'='permark'
'suet' enclosing 'permark' is 'SUPERMARKET'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for supermarket that I've seen before include "Large store selling food and household supplies" , "Large retail outlet" , "Large shop" , "Grocery store" .)