Roof worker is the one criticising (6)
I believe the answer is:
slater
I believe this clue is a double definition.
'roof worker' is the first definition.
(I've seen this before)
'the one criticising' is the second definition.
('slate' can mean to severely criticise)
'is' is the link.
(Other definitions for slater that I've seen before include "Craftsman" , "Alerts (anag) - building worker" , "Roof-worker" , "High-up person" , "Layer of roof slabs" .)