Agent, lively and popular, shows profit after costs (3,4)
I believe the answer is:
net gain
'profit after costs' is the definition.
(I've seen this in another clue)
'agent lively and popular' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'agent' is 'netga' which is located in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
A single letter 'i' remains which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
'shows' acts as a link.
Can you help me to learn more?
(Another definition for net gain that I've seen is " Profit".)