Temporary retail outlets in post offices enthralling youngster (3-3)
I believe the answer is:
pop-ups
'temporary retail outlets' is the definition.
(a pop-up is a temporary shop)
'post offices enthralling youngster' is the wordplay.
'post offices' becomes 'pos' (PO is an abbreviation for post office).
'enthralling' indicates putting letters inside.
'youngster' becomes 'pup' (I've seen this in other clues).
'pos' enclosing 'pup' is 'POP-UPS'.
'in' is the link.
(Another definition for pop-ups that I've seen is " Children's books and temporary shops".)
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