Brill for each coming aboard U-boat (6)
I believe the answer is:
superb
'brill' is the definition.
(brill is short for brilliant)
'each coming aboard u-boat' is the wordplay.
'each' becomes 'per'.
'coming aboard' means one lot of letters goes inside another.
'u-boat' becomes 'sub' (synonyms).
'per' going into 'sub' is 'SUPERB'.
'for' is the link.
(Other definitions for superb that I've seen before include "Splendid, brilliant" , "Terrific" , "First-rate" , "Outstandingly excellent" , "Tiptop" .)