U-boat half sunk at Beachy Head (3)
I believe the answer is:
sub
'u-boat' is the definition.
(German submarine)
'half sunk at beachy head' is the wordplay.
'half' means to take half the letters.
'at' is a charade indicator (letters next to each other).
'head' suggests taking the first letters.
The first letter of 'beachy' is 'b'.
'sunk' halved is 'su'.
'su'+'b'='SUB'
(Other definitions for sub that I've seen before include "Prefix meaning under or inferior" , "Reserve (abbr)" , "Navy vessel" , "Contribution; a boat" , "Advance payment" .)