Posh nuts in store (4)
I believe the answer is:
shop
'store' is the definition.
(synonyms)
'posh nuts' is the wordplay.
'nuts' indicates an anagram (nuts can mean crazy).
'posh' anagrammed gives 'SHOP'.
'in' acts as a link.
(Other definitions for shop that I've seen before include "Where goods are bought and sold" , "Be disloyal to" , "Emporium" , "Betray - place selling things" , "Establishment for the sale of goods" .)