Plan article supporting freshwater fish (4)
I believe the answer is:
idea
'plan' is the definition.
(both can mean an intention)
'article supporting freshwater fish' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'freshwater fish' becomes 'ide' (type of fish).
'a' put after 'ide' is 'IDEA'.
(Other definitions for idea that I've seen before include "Clue" , "Product of the mind" , "Brainwave" , "Fancy" , "Theory" .)