Fish dish containing potassium (4)
I believe the answer is:
pike
'fish' is the definition.
(pike is a kind of fish)
'dish containing potassium' is the wordplay.
'dish' becomes 'pie' (I've seen this before).
'containing' is an insertion indicator.
'potassium' becomes 'k' (K is the chemical symbol for potassium).
'pie' placed around 'k' is 'PIKE'.
(Other definitions for pike that I've seen before include "Fish - medieval weapon" , "may be hooked" , "Fresh-water fish or old spear" , "Old spear or voracious lake fish" , "Old weapon -- Dad's Army character" .)