Mash is prepared to accompany one fish dish (7)
I believe the answer is:
sashimi
'fish dish' is the definition.
(I've seen this in another clue)
'mash is prepared to accompany one' is the wordplay.
'prepared' indicates an anagram.
'to accompany' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'mash'+'is'='mashis'
'mashis' anagrammed gives 'sashim'.
'sashim'+'i'='SASHIMI'
(Other definitions for sashimi that I've seen before include "food in Japan" , "fish food?" , "Japanese dish of thinly sliced raw fish" , "foreign dish" , "Japanese raw fish dish - his aims (anag)" .)