It's rude from unheralded composer (6)
I believe the answer is:
handel
'it's' is the definition.
I can't judge whether this definition defines the answer.
'rude from unheralded composer' is the wordplay.
'from' is a deletion indicator.
'composer' indicates anagramming the letters.
'unheralded' with 'rude' taken away is 'nhaled'.
'nhaled' with letters rearranged gives 'HANDEL'.
Can you help me to learn more?
(Other definitions for handel that I've seen before include "German composer, lived in England" , "George Frideric --, composer" , "Old musician" , "German composer, George Frederick (1685-1759)" , "German composer of Messiah" .)