Sailor escaping bottom of ocean for a start (4)
I believe the answer is:
seed
'a start' is the definition.
(I've seen this in another clue)
'sailor escaping bottom of ocean' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'escaping' suggests deleting specific letters.
'bottom of ocean' becomes 'seabed' (I've seen this before).
'seabed' with 'ab' taken away is 'SEED'.
'for' acts as a link.
(Other definitions for seed that I've seen before include "See 19" , "Top Wimbledon player" , "Germ; beginning" , "Race" , "What is sown" .)