Half-hearted shout, not on deck (5)
I believe the answer is:
below
'not on deck' is the definition.
(ie below deck)
'half-hearted shout' is the wordplay.
'half' becomes 'bel' (I can't explain this - if you can you should give a lot more credence to this answer).
'hearted' indicates the central letters.
'shout' becomes 'yowl' (yowl is a kind of shout).
The centre of 'yowl' is 'ow'.
'bel'+'ow'='BELOW'
(Other definitions for below that I've seen before include "Feel miserable" , "Beneath" , "Inferior" , "in one's cabin" , "Later in text" .)