Disinfectant is no help in fever (6)
I believe the answer is:
phenol
'disinfectant' is the definition.
(I've seen this before)
'no help in fever' is the wordplay.
'in fever' indicates anagramming the letters (a feverish or unwell version of the letters).
'no'+'help'='nohelp'
'nohelp' is an anagram of 'PHENOL'.
'is' acts as a link.
(Other definitions for phenol that I've seen before include "cleaner" , "Organic compound; no help (anag.)" , "Acidic substance" , "Carbolic acid" , "Powerful disinfectant" .)