Darkness before navy's boat returns (6)
I believe the answer is:
erebus
'darkness' is the definition.
(personification of darkness in Greek myth)
'before navy's boat returns' is the wordplay.
'before' becomes 'ere' (usually literary).
'navy's boat' becomes 'sub' (submarine).
'returns' says the letters should be written in reverse.
'sub' reversed gives 'bus'.
'ere'+'bus'='EREBUS'
(Other definitions for erebus that I've seen before include "God of darkness - cavern on the way to Hades - active volcano in Antarctica" , "Be sure (anag) - Greek god" , "Volcanic mountain in Antarctica" , "Level of the underworld" , "Greek god of darkness, son of Chaos" .)