Rust fungus (5)
I believe the answer is:
uredo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rust fungus</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for uredo that I've seen before include "Rust fungus" , "rust, in the garden" .)