United with one sailor - in essence, a musician (9)
I believe the answer is:
guitarist
'a musician' is the definition.
(I know that guitarist is a type of musician)
'united with one sailor in essence' is the wordplay.
'united with' becomes 'ui' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one sailor' becomes 'tar' (slang term for a sailor).
'in' indicates putting letters inside.
'essence' becomes 'gist' (synonyms).
'ui'+'tar'='uitar'
'uitar' placed inside 'gist' is 'GUITARIST'.
Can you help me to learn more?
(Other definitions for guitarist that I've seen before include "String player" , "Axeman" , "Player of a particular stringed instrument" , "Player of a fretted instrument" , "Musician" .)