Head off a fuzzy fever (4)
I believe the answer is:
ague
'fever' is the definition.
(I've seen this in another clue)
'head off a fuzzy' is the wordplay.
'head off' means to remove the first letter.
'a fuzzy' becomes 'vague' (I've seen this in another clue).
'vague' with its first letter taken away is 'AGUE'.
(Other definitions for ague that I've seen before include "Feverish chill" , "met with cheek in Twelfth Night" , "attack" , "Illness with fever and shivering, malaria say" , "Fit of the shakes" .)