Tweet a quick 'Cheer up!' (7)

I believe the answer is:

chirrup

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

'tweet' is the definition.
(I know that tweet can be written as chirrup)

'a quick cheer up' is the wordplay.
'a quick cheer' becomes 'chirr' (I can't justify this - if you can you should give a lot more credence to this answer).
'chirr'+'up'='CHIRRUP'

Can you help me to learn more?

(Other definitions for chirrup that I've seen before include "Cheep" , "Chirp" , "Bird's noise" , "Sound made by a bird" , "Noise of a small bird" .)

I've seen this clue in the Sydney Morning Herald.

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