A magpie, for example, heard devouring small cuckoo (6)
I believe the answer is:
absurd
'heard devouring small cuckoo' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives can occasionally mean the same thing.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for absurd that I've seen before include "Ridiculous, quite nonsensical" , "Ridiculous, ludicrous" , "Incredible" , "Ludicrous, silly" , "Derisory, preposterous" .)