He observes an expert on land measurement (8)
I believe the answer is:
surveyor
This could be a double definition.
'he observes an' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'expert on land measurement' is the second definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps there's a link between them I don't understand?
Can you help me to learn more?
(Other definitions for surveyor that I've seen before include "Land or building measurer" , "One determines boundaries, elevations etc" , "Examiner and map-maker of lands and buildings" , "Official inspector" , "Building examiner" .)