Getting on one's high horse about a painting (8)
I believe the answer is:
mounting
'getting on' is the definition.
('mount' can be a synonym of 'get on')
'one's high horse about a painting' is the wordplay.
I cannot quite see how this works, but
'horse' could be 'mount' (mount is a kind of horse) and 'mount' is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for mounting that I've seen is " Increasing".)