The most common and most likely answer to this crossword clue is the 11 letter word STOREKEEPER
I've seen this crossword clue in The Times
S
T
O
R
E
K
E
E
P
E
R
More clues leading to the result STOREKEEPER
Buddy explains!

The straight part
We know that "Retailer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case retailer = storekeeper.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does STOREKEEPER mean?
STOREKEEPER noun- One who runs a shop, either the owner or manager.
- One who is in charge of stores or goods of any kind.
- Any unsaleable item.