American market having number joining queue initially outside supermarket (6)
I believe the answer is:
nasdaq
'american market' is the definition.
(American stock exchange)
'number joining queue initially outside supermarket' is the wordplay.
'number joining' becomes 'n' (I am not sure about this - if you are sure you should believe this answer much more).
'initially' indicates taking the first letters.
'outside' is an insertion indicator.
'supermarket' becomes 'Asda' (Asda is a supermarket).
The initial letter of 'queue' is 'q'.
'n'+'q'='nq'
'nq' placed around 'asda' is 'NASDAQ'.
'having' acts as a link.
(Other definitions for nasdaq that I've seen before include "[STOCK EXCHANGE]" , "American stock market" , "Computerised US stock market" .)