In theory, one supports fish? (7)
I believe the answer is:
ideally
'in theory' is the definition.
(I've seen this before)
'one supports fish?' is the wordplay.
'one' becomes 'ally' (I am not sure about this - if you are sure you should believe this answer much more).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'fish?' becomes 'ide' (type of fish).
'ally' put after 'ide' is 'IDEALLY'.
Can you help me to learn more?
(Other definitions for ideally that I've seen before include "for preference" , "Perfectly; most suitably" , "In the most perfect way" , "In a perfect world, the best possible situation" , "for best results" .)