One not viewed professionally in the main? (10)
I believe the answer is:
submariner
'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'not viewed professionally in the main?' is the wordplay.
I cannot really see how this works, but
'in' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for submariner that I've seen before include "one under?" , "Rum Serbian? (anag)" , "Sailor" , "Naval crewmember" .)