British sailor heading for port - hours at sea crossing from Europe to Asia (9)
I believe the answer is:
bosphorus
'crossing from europe to asia' is the definition.
(strait between Europe and Asia)
'british sailor heading for port hours at sea' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'sailor' becomes 'os' (ordinary seaman).
'heading for' suggests taking the first letters.
'at sea' indicates an anagram.
The first letter of 'port' is 'p'.
'hours' with letters rearranged gives 'horus'.
'b'+'os'+'p'+'horus'='BOSPHORUS'
(Other definitions for bosphorus that I've seen before include "Strait near Black Sea" , "Stretch of water" , "Strait straddled by Istanbul" , "Strait dividing Turkey" , "Strait separating parts of Turkey" .)