Those last in races at sea may remain behind (4)
I believe the answer is:
stay
'remain behind' is the definition.
(synonyms)
'those last in races at sea may' is the wordplay.
'those last in' indicates one should take the final letters (the letters coming last in the words).
The final letters of 'races at sea may' are 'STAY'.
(Other definitions for stay that I've seen before include "Thin strip used to stiffen a garment" , "remain in the same place" , "guy to > put off" , "Remain behind" , "Remain; prop" .)