It flies up and down (3)

I believe the answer is:

tit

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'up and down' is the definition.

'it flies' is the wordplay.
'it' becomes 't' (abbreviation. e.g. in 'tis).
'flies' becomes 'it' (I can't justify this - if you can you should believe this answer much more).
't'+'it'='TIT'

Can you help me to learn more?

(Other definitions for tit that I've seen before include "Small songbird" , "18's [BIRD]" , "Small bird that lives off insects" , "useless horse, or not?" , "Small garden bird" .)

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