Unnecessary warning to a caretaker? (7)

I believe the answer is:

caution

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

'unnecessary warning' is the definition.
'caution' can be an answer for 'warning' (caution is a kind of warning). I'm unsure of the 'unnecessary' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for caution that I've seen before include "Heed - warning - funny person" , "Legal warning" , "Prudent wariness" , "Care taken to avoid danger" , "Admonish - warning" .)

I've seen this clue in The Globe and Mail.

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