It lays eggs in the nests of others (6)
I believe the answer is:
cuckoo
'it lays eggs in the nests of others' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cuckoo that I've seen before include "Unwelcome bird in the nest" , "sound? Hardly!" , "slightly mad type" , "Doolally - bird" , "Slightly mad - bird" .)