Expert and in good condition, gaining advantage (6)
I believe the answer is:
profit
'advantage' is the definition.
(I know that profit is a type of advantage)
'expert and in good condition' is the wordplay.
'expert' becomes 'pro' (short for professional).
'and' means one lot of letters go next to another.
'in good condition' becomes 'fit' (I've seen this in another clue).
'pro'+'fit'='PROFIT'
'gaining' is the link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for profit that I've seen before include "Use" , "Clean up" , "Money made" , "Yield - gain" , "It's gained on a transaction" .)