Leave without it and midshipman gets sick (6)
I believe the answer is:
queasy
'sick' is the definition.
('queasy' can be similar in meaning to 'sick')
I don't understand the rest of the clue.
(Other definitions for queasy that I've seen before include "Squeamish - nauseous" , "Sickish" , "Feeling nauseous" , "Feeling slightly sick, maybe with indigestion" , "Nauseous or bilious" .)