Fever is not initially indistinct (4)
I believe the answer is:
ague
'fever' is the definition.
(I've seen this in another clue)
'not initially indistinct' is the wordplay.
'not initially' means to remove the first letter.
'indistinct' becomes 'vague' (I've seen this before).
'vague' with its initial letter taken away is 'AGUE'.
'is' acts as a link.
(Other definitions for ague that I've seen before include "Fit of the shakes" , "Fever (archaic)" , "attack" , "Malarial fever; chill" , "Acute fever; shivering fit" .)