People who won't believe a good lad returns and twitches (9)
I believe the answer is:
agnostics
'people who won't believe' is the definition.
'agnostics' can be an answer for 'people' (agnostic is a kind of person). I'm not sure about the remainder of the definition.
'a good lad returns and twitches' is the wordplay.
'good' becomes 'g' (abbreviation).
'lad' becomes 'son' (lad can informally mean a young boy).
'returns' shows that the letters should be reversed in order.
'and' is a charade indicator (letters next to each other).
'twitches' becomes 'tics' (tic is a kind of twitch).
'son' reversed gives 'nos'.
'a'+'g'+'nos'+'tics'='AGNOSTICS'
Can you help me to learn more?
(Other definitions for agnostics that I've seen before include "They refuse to believe" , "Sceptics" , "They don't know if God exists or not" , "They claim one can't have true knowledge of the existence of God" .)