View old shackle (7)
I believe the answer is:
opinion
'view' is the definition.
('opinion' can be a synonym of 'view')
'old shackle' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'shackle' becomes 'pinion' ('pinion' can be a synonym of 'shackle').
'o'+'pinion'='OPINION'
(Other definitions for opinion that I've seen before include "school of thought" , "Viewpoint" , "Belief or conviction" , "Unproven belief" , "Assessment" .)