Bird embarks on harmless piece of mischief (4)

I believe the answer is:

lark

I'm a little stuck... Click here to teach me more about this clue!

'harmless piece of mischief' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for lark that I've seen before include "One in exaltation" , "Mischievous game" , "Singing bird and piece of harmless mischief" , "Flighty type" , "High-flying songbird" .)

I've seen this clue in The Irish Times.

Want a hint initially instead of a full solution? Install my app