Prepare to sail vessel, cape appearing in hour at sea (2-6)
I believe the answer is:
up-anchor
'prepare to sail' is the definition.
(I've seen this in another clue)
'vessel cape appearing in hour at sea' is the wordplay.
'vessel' becomes 'pan' (vessel or container for cooking).
'cape' becomes 'c' (abbreviation used on maps).
'appearing in' is an insertion indicator.
'at sea' indicates anagramming the letters.
'pan'+'c'='panc'
'hour' anagrammed gives 'uhor'.
'panc' going within 'uhor' is 'UP-ANCHOR'.
(Another definition for up-anchor that I've seen is " (Of a ship) get moving".)