It's not Thatcher he's criticising (6)
I believe the answer is:
slater
'it's not thatcher he's criticising' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for slater that I've seen before include "Professional roof worker" , "Does one abuse" , "Roof-worker" , "Alerts (anag) - building worker" , "Layer of roof slabs" .)