Supplier seeing Queen before King? (7)
I believe the answer is:
caterer
'supplier' is the definition.
(caterer is a kind of supplier)
'queen before king?' is the wordplay.
'queen' becomes 'cat' (a queen is a female cat).
'before' becomes 'ere' (usually literary).
'king?' becomes 'r' (r is abbreviation for rex).
'cat'+'ere'+'r'='CATERER'
'seeing' acts as a link.
(Other definitions for caterer that I've seen before include "Provision supplier" , "Person who supplies food for social events" , "One providing food and drink" , "Provider of food/drink" , "One providing food and service for a party" .)