Thought to be a disc on the wing (7)
I believe the answer is:
opinion
'thought' is the definition.
(synonyms)
'a disc on the wing' is the wordplay.
'a disc' becomes 'O' (disc-shaped letter).
'on' means one lot of letters go next to another.
'the wing' becomes 'pinion' (synonyms).
'o'+'pinion'='OPINION'
'to be' is the link.
(Other definitions for opinion that I've seen before include "Standpoint" , "A view held, what one thinks" , "Belief not founded on proof or certainty" , "Personal view" , "Viewpoint" .)