The professor takes it as net gain (6)
I believe the answer is:
profit
'net gain' is the definition.
(I've seen this before)
'the professor takes it' is the wordplay.
'the professor takes' becomes 'prof' (I can't justify this - if you can you should give a lot more credence to this answer).
'prof'+'it'='PROFIT'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for profit that I've seen before include "Net gain after expenses" , "Yield - gain" , "Use" , "financial advantage" , "returns" .)