Unreasonably riled by a slacker (5)

I believe the answer is:

idler

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

'a slacker' is the definition.
(I've seen this before)

'unreasonably riled' is the wordplay.
'riled' can be anagrammed to 'IDLER'.
Though, I'm not sure how the anagram is indicated.

'by' is the link.

Can you help me to learn more?

(Other definitions for idler that I've seen before include "Sluggard" , "comparatively shoddy?" , "Lazy person" , "One avoiding work" , "bad worker" .)

I've seen this clue in the Evening Standard.

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