Period of maritime decline (3)
I believe the answer is:
ebb
'decline' is the definition.
(I know that ebb is a type of decline)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ebb that I've seen before include "Receding movement, as of a tide" , "recession" , "state of decline" , "Type of tide" , "(Of tidewater) recede" .)