Brill for each boarding U-boat (6)
I believe the answer is:
superb
'brill' is the definition.
(brill is short for brilliant)
'each boarding u-boat' is the wordplay.
'each' becomes 'per' ('per' can mean for each).
'boarding' means one lot of letters goes inside another.
'u-boat' becomes 'sub' (German submarine).
'per' inserted into 'sub' is 'SUPERB'.
'for' acts as a link.
(Other definitions for superb that I've seen before include "First-class" , "Splendid, marvellous" , "Surpassingly good" , "Tiptop" , "Outstandingly excellent" .)