On boat, British crewman in a delta (6)
I believe the answer is:
aboard
'on boat' is the definition.
(I've seen this before)
'british crewman in a delta' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in BBC).
'crewman' becomes 'oar' (I've seen this in another clue).
'in' indicates putting letters inside.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'b'+'oar'='boar'
'a'+'d'='ad'
'boar' going inside 'ad' is 'ABOARD'.
(Other definitions for aboard that I've seen before include "on a liner, say" , "at sea, say?" , "On vessel or vehicle" , "On (a ship/plane)" , "Part of the group" .)