Fungus beginning to taint nettle (7)
I believe the answer is:
truffle
'fungus' is the definition.
(I've seen this before)
'beginning to taint nettle' is the wordplay.
'beginning to' indicates taking the first letters.
'nettle' becomes 'ruffle' (ruffling is a kind of nettling).
The first letter of 'taint' is 't'.
't'+'ruffle'='TRUFFLE'
(Other definitions for truffle that I've seen before include "Fungoid delicacy" , "Valued culinary fungus" , "Witty remark" , "Earthnut" , "Fretful for tasty underground fungus" .)