Prepare fish? Fresh eels and dace, mostly (7)
I believe the answer is:
descale
'prepare fish?' is the definition.
(remove scales from fish)
'fresh eels and dace mostly' is the wordplay.
'fresh' indicates anagramming the letters.
'and' says to put letters next to each other.
'mostly' means to remove the last letter.
'dace' with its final letter taken off is 'dac'.
'eels'+'dac'='eelsdac'
'eelsdac' is an anagram of 'DESCALE'.
(Other definitions for descale that I've seen before include "Remove lime deposit" , "Remove lime (from kettle, boiler etc.)" .)