One of the birds or bats (6)
I believe the answer is:
cuckoo
'bats' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for cuckoo that I've seen before include "Bird (with a bad reputation)" , "nuts" , "Hourly you might hear this" , "Slightly mad - bird" , "Bird which lays eggs in others' nests" .)