Upset if gun rusts? (5)
I believe the answer is:
fungi
'rusts?' is the definition.
(rust is a kind of fungus)
'upset if gun' is the wordplay.
'upset' is an anagram indicator.
'if'+'gun'='ifgun'
'ifgun' is an anagram of 'FUNGI'.
(Other definitions for fungi that I've seen before include "Spore-producing organisms" , "Eg, toadstools" , "Mushrooms, toadstools, moulds etc" , "Spore producers" , "Organisms studied by a mycologist" .)