Arrest felon at last, and be sick (4)
I believe the answer is:
nail
'arrest' is the definition.
(I know that arrest can be written as nail)
'felon at last and be sick' is the wordplay.
'at last' indicates one should take the final letters.
'and' is a charade indicator (letters next to each other).
'be sick' becomes 'ail' (synonyms).
The last letter of 'felon' is 'n'.
'n'+'ail'='NAIL'
(Other definitions for nail that I've seen before include "Thin pointed fastener" , "Apprehend" , "one wouldn't want in one's coffin?" , "Fixer (on finger?)" , "Hit this on the head to be dead right" .)