Heavyweight's heading to meet Rocky Marciano -- one might blow it (9)
I believe the answer is:
harmonica
'blow it' is the definition.
'heavyweight's heading to meet rocky marciano' is the wordplay.
'heading to' indicates taking the first letters.
'meet' means one lot of letters go next to another.
'rocky' indicates an anagram.
The first letter of 'heavyweights' is 'h'.
'marciano' anagrammed gives 'armonica'.
'h'+'armonica'='HARMONICA'
'one might' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for harmonica that I've seen before include "Mouth organ" , "It's blown" , "musical instrument" , "glasses of water" , "Larry Adler's instrument" .)