Heavens, how quickly a chocolate bar goes (9)
I believe the answer is:
aerospace
'heavens how quickly a chocolate' is the definition.
I can't judge whether this definition defines the answer.
'bar goes' is the wordplay.
'bar' becomes 'aero' (chocolate bar).
'goes' becomes 'space' (I can't explain this - if you can you should give a lot more credence to this answer).
'aero'+'space'='AEROSPACE'
Can you help me to learn more?
(Other definitions for aerospace that I've seen before include "Opera case (anag)" , "Industry of flight" , "Nasa's speciality" , "rocket industry" , "Branch of technology including aviation" .)