In a perfect world, fish with friend (7)
I believe the answer is:
ideally
'in a perfect world' is the definition.
(in an ideal world)
'fish with friend' is the wordplay.
'fish' becomes 'ide' (type of fish).
'with' means one lot of letters go next to another.
'friend' becomes 'ally' (an ally is a friend or supporter).
'ide'+'ally'='IDEALLY'
(Other definitions for ideally that I've seen before include "Lay idle (anag)" , "In the best of all possible worlds" , "in optimal way" , "for preference" , "In the most perfect way" .)