Shivering fit is unclear, but not initially (4)
I believe the answer is:
ague
'shivering fit' is the definition.
(I've seen this in another clue)
'unclear but not initially' is the wordplay.
'unclear' becomes 'vague' (similar in meaning).
'but not initially' means to remove the first letter.
'vague' with its first letter taken off is 'AGUE'.
'is' is the link.
(Other definitions for ague that I've seen before include "Hot and cold fever" , "attack" , "Feverish chill" , "Complaint" , "Malaria, for example" .)