Complaint is indistinct, but not initially (4)
I believe the answer is:
ague
'complaint' is the definition.
(ague is a kind of health problem)
'indistinct but not initially' is the wordplay.
'indistinct' becomes 'vague' (I've seen this in another clue).
'but not initially' means to remove the first letter.
'vague' with its initial letter taken off is 'AGUE'.
'is' acts as a link.
(Other definitions for ague that I've seen before include "Illness with fever and shivering, malaria say" , "Old word for fever" , "Fit of shivering and shaking" , "Sweating, shivering, fit (once)" , "met with cheek in Twelfth Night" .)