Personal belief of a duck on the wing? (7)
I believe the answer is:
'personal belief' is the definition.
(I've seen this before)
'a duck on the wing?' is the wordplay.
'a duck' becomes 'o' (resembles zero - 'duck' in cricket).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'the wing?' becomes 'pinion' ('pinion' can be a synonym of 'wing').
'of' acts as a link.
(Other definitions for opinion that I've seen before include "'Judgment, view (7)'" , "Unproved belief or view" , "Assessment" , "Standpoint" , "Personal belief without proof" .)