He's very critical of the building worker (6)
I believe the answer is:
slater
This clue might be a double definition.
'he's very critical' is the first definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'building worker' is the second definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'of the' acts as a link.
Can you help me to learn more?
(Other definitions for slater that I've seen before include "Man on top" , "High-up person" , "Professional roof worker" , "Layer of roof slabs" , "Roof-worker" .)