Opening answer cheeky: you're heard (8)
I believe the answer is:
aperture
'opening' is the definition.
(I know that aperture is a type of opening)
'answer cheeky you're heard' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'cheeky' becomes 'pert' (pert can mean cheeky or insolent).
'you're' becomes 'ur' (I am not sure about this - if you are sure you should believe this answer much more).
'heard' becomes 'e' (I've seen this in other clues).
'a'+'pert'+'ur'+'e'='APERTURE'
Can you help me to learn more?
(Other definitions for aperture that I've seen before include "Device controlling amount of light admitted" , "Eye" , "Crack" , "Opening through which light enters a camera" , "Small hole or opening" .)