One criticising alternative to Thatcher? (6)

I believe the answer is:

slater

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'one criticising' is the definition.
('slate' can mean to severely criticise)

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

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(Other definitions for slater that I've seen before include "Man on top" , "Roof-worker" , "Professional roof worker" , "High-up person" , "High-reaching worker perhaps" .)

I've seen this clue in The Times.

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