One breaking out with key — a whizz! (7)
I believe the answer is:
escapee
'whizz' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'one breaking out with key a' is the wordplay.
'one breaking out' becomes 'pee' (I am not sure about this - if you are sure you should believe this answer much more).
'with' means one lot of letters go next to another.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'esc'+'a'='esca'
'pee' after 'esca' is 'ESCAPEE'.
Can you help me to learn more?
(Other definitions for escapee that I've seen before include "One breaking free" , "One that's broken free" , "Jail-breaker" , "Runaway" , "One absconding" .)