Vessel to let loose at Beachy Head (6)
I believe the answer is:
bottle
'vessel' is the definition.
(bottle is a kind of vessel)
'to let loose at beachy head' is the wordplay.
'loose' is an anagram indicator (letters are loosened from their original positions).
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'head' says to take the initial letters.
The initial letter of 'beachy' is 'b'.
'to'+'let'='tolet'
'tolet' with letters rearranged gives 'ottle'.
'ottle' after 'b' is 'BOTTLE'.
(Other definitions for bottle that I've seen before include "The spirit" , "Type of marine mammal" , "Nerve - container" , "cool" , "Glass container or slangy courage" .)