He has absolute control of the French vessel (6)
I believe the answer is:
despot
'he has absolute control of' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'french vessel' is the wordplay.
'french' becomes 'des' (I can't explain this - if you can you should believe this answer much more).
'vessel' becomes 'pot' (type of container).
'des'+'pot'='DESPOT'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for despot that I've seen before include "Cruel and oppressive dictator" , "Arbitrary ruler" , "has first of shocks in store" , "Posted by tyrant" , "Absolute ruler, tyrant" .)