Sea may do this to ship (4)
I believe the answer is:
'sea may do' is the definition.
The definition and answer can be both to do with motion as well as being verbs in their base form.
Maybe you can see a link between them that I can't see?
'to ship' is the wordplay.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'this' acts as a link.
Can you help me to learn more?
(Other definitions for toss that I've seen before include "Spin of a coin" , "Throw (coin or caber)" , "Ditch" , "Raise a little money" , "when ship does this?" .)