Work in one, on view (7)
I believe the answer is:
opinion
'view' is the definition.
(I know that view can be written as opinion)
'work in one on' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'one' becomes 'I' (Roman numeral).
'op'+'in'+'i'+'on'='OPINION'
(Other definitions for opinion that I've seen before include "Standpoint" , "Viewpoint" , "thinking" , "A view held as probable - well, that's mine" , "Assessment" .)