Take on old gear (7)
I believe the answer is:
opinion
'take' is the definition.
(I've seen this in another clue)
'on old gear' is the wordplay.
'on' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'gear' becomes 'pinion' (pinion is a kind of gear).
'o' put next to 'pinion' is 'OPINION'.
(Other definitions for opinion that I've seen before include "Unproven belief" , "'Judgment, view (7)'" , "Viewpoint" , "View, barrister's advice" , "Personal belief without proof" .)