Rust fungus in cultured output (5)
I believe the answer is:
Here is my best explanation:
'rust fungus' is the definition.
(I've seen this before)
'in cultured output' is the subsidiary indication.
'in' says the answer is hidden in the clue.
'uredo' can be found hidden inside 'cultured output'.
(Other definitions for uredo that I've seen before include "rust, in the garden" and "Rust fungus".)