Have a quiet beer then rock! (5)
I believe the answer is:
shale
'rock' is the definition.
(shale is a kind of rock)
'have a quiet beer' is the wordplay.
'have a quiet' becomes 'sh' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'beer' becomes 'ale' (type of beer).
'sh'+'ale'='SHALE'
'then' is the link.
Can you help me to learn more?
(Other definitions for shale that I've seen before include "Layered sedimentary rock" , "Oil-rich rock" , "Soft sedimentary rock" , "Easily-split sedimentary rock" , "Stone that splits easily into fine pieces" .)