Mushroom I cooked initially with a fish (6)
I believe the answer is:
agaric
'mushroom' is the definition.
(I've seen this before)
'i cooked initially with a fish' is the wordplay.
'initially' indicates taking the first letters.
'with' says to put letters next to each other.
'fish' becomes 'gar' (type of gar).
The first letter of 'cooked' is 'c'.
'i'+'c'='ic'
'a'+'gar'='agar'
'ic' put after 'agar' is 'AGARIC'.
(Other definitions for agaric that I've seen before include "Type of mushroom" , "Fungus, eg, mushroom" , "fungal growth" , "Fungus of mushroom family" .)