This is how leading expert, in conclusion, waxed lyrical (8)
I believe the answer is:
enthused
'waxed lyrical' is the definition.
'this is how leading expert in conclusion' is the wordplay.
'this is how' becomes 'thus' (I can't explain this - if you can you should give a lot more credence to this answer).
'leading' says to take the initial letters.
'in' means one lot of letters goes inside another.
'conclusion' becomes 'end' (something's conclusion is its end).
The initial letter of 'expert' is 'e'.
'thus'+'e'='thuse'
'thuse' placed within 'end' is 'ENTHUSED'.
Can you help me to learn more?
(Other definitions for enthused that I've seen before include "The nudes (anag)" , "Showed great interest" , "Jelly-like sweet" , "Made extremely interested" , "Showed eager liking for or interest in" .)