Fool Louis Armstrong forgetting a finale for trumpet! (5)
I believe the answer is:
schmo
'fool' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for schmo that I've seen before include "jerk" , "clot" , "Fool (Yiddish)" , "Inept person (US_informal)" , "Boring American" .)