Major's really sick (8)
I believe the answer is:
critical
I believe this is a double definition.
'major's' is the first definition.
(I've seen this before)
'really sick' is the second definition.
(as in 'critically ill')
(Other definitions for critical that I've seen before include "giving literary analysis" , "Urgent - fault-finding" , "Finding fault - at a dangerous stage" , "serious" , "Major" .)