Quiet bit of lunch? Avoid it! (4)
I believe the answer is:
shun
'lunch? avoid it' is the definition.
I can't tell whether this defines the answer.
'quiet bit' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'bit' becomes 'un' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sh'+'un'='SHUN'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for shun that I've seen before include "Ostracise" , "Deliberately have no contact with" , "military command" , "Avoid - attention!" , "Parade-ground language" .)