Fit is imprecise, very lacking (4)
I believe the answer is:
ague
'fit' is the definition.
(I've seen this before)
'imprecise very lacking' is the wordplay.
'imprecise' becomes 'vague' ('vague' can be a synonym of 'imprecise').
'very' becomes 'v' (abbreviation).
'lacking' indicates named letters should be taken away.
'vague' with 'v' taken away is 'AGUE'.
'is' is the link.
(Other definitions for ague that I've seen before include "Sweating, shivering, fit (once)" , "Fit of the shakes" , "Malarial fever or fit of shivering" , "Illness with fever and shivering, malaria say" , "Old word for fever" .)