Under the weather (3)
I believe the answer is:
'under the weather' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ill that I've seen before include "I will (abbrev.) (1'2)" , "Badly - in need of treatment" , "Shakespeare: '. . . met by moonlight'" , "Unwell from bacillus" , "Sickly" .)