Source of this disorder is fungus (7)
I believe the answer is:
truffle
'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'
(Other definitions for truffle that I've seen before include "Sweet - edible fungus growing underground" , "Witty remark" , "Edible fungus -- chocolate sweet" , "Rich food" , "Edible subterranean fungus" .)