I will be unwell (3)
I believe the answer is:
ill
This clue is a double definition.
'i will' is the first definition.
(abbreviation)
'unwell' is the second definition.
(similar in meaning)
'be' is the link.
(Other definitions for ill that I've seen before include "Ailing" , "Off colour?" , "state, briefly" , "Infirm" , "Dicky" .)