Now likely later to become ill (2,3,4)
I believe the answer is:
on the nail
'now' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'likely later to become ill' is the wordplay.
'likely' becomes 'on' (I've seen this in other clues).
'later' becomes 'then' (I've seen this in other clues).
'to become ill' becomes 'ail'.
'on'+'then'+'ail'='ON THE NAIL'
Can you help me to learn more?
(Other definitions for on the nail that I've seen before include "without delay" , "location of solution for digital protection" , "focus of manicurist?" , "Where varnish is put" , "Where a coat may be hung" .)