Can Jack be sick? (4)
I believe the answer is:
jail
'can' is the definition.
(The Can is slang for prison)
'jack be sick?' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'be sick?' becomes 'ail' ('ail' can be a synonym of 'be sick'**).
'j'+'ail'='JAIL'
(Other definitions for jail that I've seen before include "sentence" , "(Send to) prison" , "Clink" , "having a stretch here?" , "Cooler" .)