Rook is served with fish dish (7)
I believe the answer is:
rissole
'dish' is the definition.
(rissole is a kind of dish)
'rook is served with fish' is the wordplay.
'rook' becomes 'ris' (I can't explain this - if you can you should give a lot more credence to this answer).
'is served with' says to put letters next to each other.
'fish' becomes 'sole' (sole is a kind of fish).
'ris'+'sole'='RISSOLE'
Can you help me to learn more?
(Other definitions for rissole that I've seen before include "Meatball fried in crumbs" , "Fried meat dish" , "Lorises (anag)" , "Meat product" , "Minced meat or fish deep fried in egg and breadcrumbs" .)