He takes part in naval operations (5,6)
I believe the answer is:
ships doctor
'naval operations' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'he takes part' is the wordplay.
I cannot quite see how this works, but
'he' could be 'hi' and 'hi' is located in the answer.
'part' could be 'do' (do is a kind of part) and 'do' is located in the answer.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Another definition for ships doctor that I've seen is " medical man".)