Possible Clues:

• Bird; sort of clock
• Bird taken out to lunch?
• Bird which lays eggs in others' nests
• One had the end of 21 across but not the beginning of it
• Mad - layer of eggs in another bird's nest
• Layer of eggs in another bird's nest
• Crazy (colloq.); bird
• It lays eggs in another's nest
• The bird that invades another's nest
• Chef, it's said, loves nuts