Throw aboard ship (4)
I believe the answer is:
toss
'throw' is the definition.
(toss is a kind of throw)
'aboard ship' is the wordplay.
'aboard' becomes 'to' (I can't explain this - if you can you should believe this answer much more).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'to'+'ss'='TOSS'
Can you help me to learn more?
(Other definitions for toss that I've seen before include "Pitch" , "decision-making process" , "'Flip, a coin or pancake say (4)'" , "cast" , "Do it with caber in Scotland" .)