His vessel had a full complement of mates (4)
I believe the answer is:
noah
'his vessel had a full complement' is the definition.
The definition suggests a singular noun which matches the answer.
'of mates' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is found in the answer.
The remaining letters 'nah' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for noah that I've seen before include "Animal rescuer" , "The first known boat builder (4)" , "zoo keeper" , "Ark wright" , "biblical character" .)