Time to admit, you’ll be called fit (4)
I believe the answer is:
ague
'called fit' is the definition.
'ague' can be an answer for 'fit' (ague is a fit of fever). I'm unsure of the 'called' bit.
'time to admit you'll' is the wordplay.
'time' becomes 'age' (an age is a period of time).
'to admit' is an insertion indicator.
'you'll' becomes 'u' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'age' enclosing 'u' is 'AGUE'.
'be' is the link.
Can you help me to learn more?
(Other definitions for ague that I've seen before include "Sickness" , "Old word for fever" , "Feverish illness" , "Burning fever" , "Acute fever; shivering fit" .)