Having turned up, adds water to casserole (4)
I believe the answer is:
stew
'casserole' is the definition.
(I've seen this before)
'having turned up adds water to' is the wordplay.
'having turned up' says the letters should be written backwards.
'adds water to' becomes 'wets' (to wet something is to add water to it).
'wets' reversed gives 'STEW'.
(Other definitions for stew that I've seen before include "Cook by long slow simmering" , "Artificial oyster bed" , "Cassoulet" , "Slow-cooked dish of meat and vegetables" , "Simmer" .)