Ships move under it (3)
I believe the answer is:
way
'ships move' is the definition.
'way' can be an answer for 'move' (way is a kind of move). I'm unsure of the 'ships' bit.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for way that I've seen before include "Street" , "'Manner, mode (3)'" , "Route; direction" , "Method; route" , "Manner; road" .)