I will become unwell
I believe the answer is:
ill
This clue is a double definition.
'i will' is the first definition.
(I've seen this before)
'unwell' is the second definition.
(similar in meaning)
'become' acts as a link.
(Other definitions for ill that I've seen before include "laid up?" , "Faulty" , "Valetudinarian" , "Not well, sick" , "Hardly - sick" .)