Fungus ruined a cigar (6)
I believe the answer is:
agaric
'fungus' is the definition.
(I know that agaric is a type of fungus)
'ruined a cigar' is the wordplay.
'ruined' indicates an anagram.
'a'+'cigar'='acigar'
'acigar' with letters rearranged gives 'AGARIC'.
(Other definitions for agaric that I've seen before include "fungal growth" , "Fungus, eg, mushroom" , "Fungus of mushroom family" , "Type of mushroom" .)