Feel miserable when not on deck? (5)
I believe the answer is:
below
I believe this is a double definition.
'feel miserable' is the first definition.
(I've seen this before)
'not on deck?' is the second definition.
(ie below deck)
'when' acts as a link.
(Other definitions for below that I've seen before include "second-rate" , "Underneath" , "Not worthy of" , "in one's cabin" , "Subordinate to" .)