Sort of critic who bangs on the door? (7)

I believe the answer is:

knocker

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

'sort of critic' is the definition.
'knocker' can be an answer for 'critic' (I've seen this before). I am unsure of the 'sort of' bit.

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

Can you help me to learn more?

(Other definitions for knocker that I've seen before include "Hinged device on a door" , "Something on door; one continually finding fault" , "Door rapper" , "Alternative to door bell" , "means to announce house call" .)

I've seen this clue in The Irish Independent.

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