Fever is indistinct, but not initially (4)
I believe the answer is:
ague
'fever' is the definition.
(I've seen this in another clue)
'indistinct but not initially' is the wordplay.
'indistinct' becomes 'vague' (I've seen this before).
'but not initially' means to remove the first letter.
'vague' with its initial letter taken away is 'AGUE'.
'is' acts as a link.
(Other definitions for ague that I've seen before include "nasty illness?" , "Malarial fever or fit of shivering" , "attack" , "Chills and fever" , "Sweating, shivering, fit (once)" .)