Offensive posters prompted by promenade cycling (6)

I believe the answer is:

trolls

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

'offensive posters' is the definition.
(people who post online in an offensive way)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for trolls that I've seen before include "They start rows on the internet" , "Ugly cave-dwelling creatures" , "Scandinavian goblins" , "Mythical Scandinavian dwarfs" , "Mythical Scandinavian cave dwellers" .)

I've seen this clue in The Guardian.

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