Start of virulent fever is uncertain (5)
I believe the answer is:
vague
'uncertain' is the definition.
(thesaurus)
'start of virulent fever' is the wordplay.
'start of' says to take the initial letters.
'fever' becomes 'ague' (I've seen this in other clues).
The initial letter of 'virulent' is 'v'.
'v'+'ague'='VAGUE'
'is' acts as a link.
(Other definitions for vague that I've seen before include "Indeterminate" , "Ill-defined" , "Not clearly expressed, woolly" , "Indistinct or not clearly expressed" , "Inexact" .)