Fail to take an opportunity, as naval defaulters may do (4,3,4)
I believe the answer is:
miss the boat
'fail' is the definition.
(I've seen this in another clue)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for miss the boat that I've seen before include "fire shot across bows, say" , "as tardy tars may do" , "can't grab it quick enough" , "Fail through lateness" , "Blow opportunity" .)