One mainly expects to see through his work (7)
I believe the answer is:
glazier
'mainly expects to see through his work' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
The remaining letters 'glazer' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for glazier that I've seen before include "Glass fitter" , "One may fix lights" , "Window mender" , "Tradesman" , "Person who cuts flat glass to size" .)