Large, top-quality mushrooms served up in dish of the day (7)
I believe the answer is:
special
'dish of the day' is the definition.
(I've seen this before)
'large top-quality mushrooms served up' is the wordplay.
'large' becomes 'l'.
'top quality' becomes 'ai' (resembles A1, informal term for top quality).
'mushrooms' becomes 'ceps' (I've seen this before).
'served up' is a reversal indicator.
'l'+'ai'+'ceps'='laiceps'
'laiceps' reversed gives 'SPECIAL'.
'in' acts as a link.
(Other definitions for special that I've seen before include "Out of the ordinary, singular" , "Though particular" , "Particularly favoured" , "Important; particular" , "Remarkable" .)