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I believe the answer is:

puffin

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

'flier' is the definition.
(puffin is a kind of birdanimal)

'advertisement featured' is the wordplay.
'advertisement' becomes 'puff' (I've seen this before).
'featured' becomes 'in' (I can't explain this - if you can you should believe this answer much more).
'puff'+'in'='PUFFIN'

'by' acts as a link.

Can you help me to learn more?

(Other definitions for puffin that I've seen before include "Seabird with a colourful bill" , "Bright-billed bird" , "Species of auk" , "Seabird (breathin' heavily?)" , "Bright-beaked sea bird" .)

I've seen this clue in The Telegraph.

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