Side-effect of rare empty drums, say (12)
I believe the answer is:
repercussion
'say' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'side-effect of rare empty drums' is the wordplay.
'side' becomes 'r' (abbreviation for right).
'effect of rare empty' becomes 'e' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'drums' becomes 'percussion'.
'r'+'e'+'percussion'='REPERCUSSION'
Can you help me to learn more?
(Other definitions for repercussion that I've seen before include "Impact" , "Unintended consequence" , "Indirect consequence of some action" , "Backlash" , "Recoil; reverberation" .)