Amateur’s debut in autumn fight? That’ll make you laugh (4,5)
I believe the answer is:
fall about
'laugh' is the definition.
(as in falling about laughing)
'amateur's debut in autumn fight?' is the wordplay.
'debut' indicates taking the first letters.
'in' indicates putting letters inside.
'autumn' becomes 'fall' (synonyms).
'fight?' becomes 'bout' (synonyms).
The initial letter of 'amateurs' is 'a'.
'fall'+'bout'='fallbout'
'a' going into 'fallbout' is 'FALL ABOUT'.
'that'll make you' acts as a link.
(Other definitions for fall about that I've seen before include "Laugh uncontrollably" , "Laugh a lot" , "Laugh heartily" .)