Crossword Genius

Source of this disorder is fungus (7)

Ross

I believe the answer is:

truffle

Here is my best explanation:

'is fungus' is the definition.
(I've seen this before)

'source of this disorder' is the wordplay.
'source of' suggests taking the first letters.
'disorder' becomes 'ruffle'.
The initial letter of 'this' is 't'.
't'+'ruffle'='TRUFFLE'

This clue was last seen in The Telegraph