Jack leaving his companion out of sorts (3)
I believe the answer is:
ill
'out of sorts' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
(Other definitions for ill that I've seen before include "Poorly" , "Out of sorts" , "Where to find Chicago" , "Where Chicagoans are briefly" , "Badly - in need of treatment" .)