Lacking definition, not very fit (4)
I believe the answer is:
ague
'fit' is the definition.
(I've seen this before)
'lacking definition not very' is the wordplay.
'lacking definition' becomes 'vague'.
'not' indicates named letters should be taken away.
'very' becomes 'v' (abbreviation).
'vague' with 'v' taken out is 'AGUE'.
(Other definitions for ague that I've seen before include "Feverish chill" , "Fever (archaic)" , "Old word for fever" , "Malaria, for example" , "Acute fever; shivering fit" .)