Sea may do this to ship (4)
I believe the answer is:
toss
'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).
'to'+'ss'='TOSS'
'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?" .)