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