Hes mainly a scientist (13)
I believe the answer is:
oceanographer
'scientist' is the definition.
(oceanographer is a kind of scientist)
'hes mainly a' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oceanographer that I've seen before include "One studying the great seas" , "Scientist concerned with the seas" , "One who studies the seas" , "Marine scientist" .)