Vessel that had a full complement of mates (3)
I believe the answer is:
ark
'vessel' is the definition.
(ark is a kind of vessel)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for ark that I've seen before include "The earliest vessel?" , "Coffer" , "Rescue boat" , "Boat built to withstand great flood" , "Noah's vessel - in which the Torah scrolls are kept" .)