Last of those aboard must stop at sea (5,2)
I believe the answer is:
heave to
'stop at sea' is the definition.
(I've seen this before)
'last of those aboard must' is the wordplay.
'last of' suggests the final letters.
'aboard' means one lot of letters goes inside another.
'must' becomes 'haveto' (I've seen this before).
The last letter of 'those' is 'e'.
'e' going within 'haveto' is 'HEAVE TO'.
(Other definitions for heave to that I've seen before include "(Of a sailing vessel) stop moving" , "stop at sea" , "come to a stop for navy" , "Of ship, stop without anchoring or mooring" .)