Sleep, as sailor in bunk (5)
I believe the answer is:
roost
'sleep' is the definition.
(for birds, to roost is to settle down to sleep)
'sailor in bunk' is the wordplay.
'sailor' becomes 'os' (ordinary seaman).
'in' indicates putting letters inside.
'bunk' becomes 'rot' (synonyms).
'os' put into 'rot' is 'ROOST'.
'as' acts as a link.
(Other definitions for roost that I've seen before include "Bird's settling place" , "Temporary resting-place" , "(Of birds) settle for rest" , "Bird's shelter" , "where fliers land" .)