He may be ill (4)
I believe the answer is:
will
'he' is the definition.
(short for William)
'be ill' is the wordplay.
'be' becomes 'w' (I can't explain this - if you can you should give a lot more credence to this answer).
'w'+'ill'='WILL'
'may' acts as a link.
Can you help me to learn more?
(Other definitions for will that I've seen before include "Last testament" , "mind" , "A traditional question from the floor?" , "Record of final wishes and testament" , "Testament or determination" .)