Fish dish a monk's seldom cooked (6,6)
I believe the answer is:
smoked salmon
'fish dish' is the definition.
(I've seen this before)
'a monk's seldom cooked' is the wordplay.
'cooked' is an anagram indicator.
'a'+'monks'+'seldom'='amonksseldom'
'amonksseldom' is an anagram of 'SMOKED SALMON'.
(Other definitions for smoked salmon that I've seen before include "fish dish" , "Fishy delicacy" , "Cured fish" , "Fish once considered terribly posh" .)