Not gross, but there's a bar on it in Barnet (3)
I believe the answer is:
net
'not gross' is the definition.
('net' is the opposite to 'gross')
'there's a bar on it in barnet' is the wordplay.
'NET' is hidden in amongst the letters.
But, I'm not sure how this is indicated.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for net that I've seen before include "Catch, ensnare" , "Final profit when all overheads have been deducted" , "Trap made from mesh" , "Make" , "Tennis court feature" .)