Twisted person (5)
'twisted person' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sicko that I've seen before include "Unpleasant character" , "pervert" , "macabre" .)
(Other definitions for sickie that I've seen before include "In day off work" , "Day off for malingerer" , "Leave taken with feigned illness" , "ill-deserved leave?" .)