Record reduction in action by mass parasite (8)
I believe the answer is:
tapeworm
'mass parasite' is the definition.
'tapeworm' can be an answer for 'parasite' (I've seen this before). I'm unsure of the 'mass' bit.
'record reduction in action by' is the wordplay.
'record' becomes 'tape record' (taping is a kind of recording).
'reduction in action' becomes 'worm' (I can't justify this - if you can you should give a lot more credence to this answer).
'by' says to put letters next to each other.
'tape'+'worm'='TAPEWORM'
Can you help me to learn more?
(Other definitions for tapeworm that I've seen before include "Sucker" , "Intestinal parasite" , "Internal parasite" , "Human parasite" , "Ribbon-like parasite inhabiting the gut" .)