Shady image, representation of Horus and Cairo with minimum of colour (11)
I believe the answer is:
chiaroscuro
'shady image' is the definition.
(term for a light and shade effect in painting)
'representation of horus and cairo with minimum of colour' is the wordplay.
'representation of' indicates an anagram (I've seen this in other clues).
'and' is a charade indicator (letters next to each other) (I've seen this in other clues).
'with' means one lot of letters goes inside another.
'minimum of' suggests taking the first letters.
The first letter of 'colour' is 'c'.
'horus' after 'cairo' is 'cairohorus'.
'cairohorus' with letters rearranged gives 'chiarosuro'.
'chiarosuro' placed around 'c' is 'CHIAROSCURO'.
(Other definitions for chiaroscuro that I've seen before include "contrast in painting" , "Caruso choir (anag) - light and shade" , "light and dark in painting" , "Black and white" , "Treatment of light and shade in drawing" .)