Image of German holding most of 6 down (6)
I believe the answer is:
Here is my best explanation:
'image' is the definition.
'of german holding most of 6 down' is the subsidiary indication.
'of german' becomes 'von' (common German surname element meaning 'of').
'holding' is an insertion indicator.
'most of' means to remove the first letter.
'6' becomes 'vi' ('VI' can be a synonym of '6').
'down' becomes 'si' (I can't justify this - if you can you should believe this answer much more).
'vi' with its first letter removed is 'i'.
'von' enclosing 'isi' is 'vision'.
(Other definitions for vision that I've seen before include "Forward plan", "The faculty of sight, may be imaginative", "Sight of an apparition?", "Sight of unusual beauty", "Perception by eye", "Apparition", "Eyesight", "Power of sight", "The power of seeing" and "Imagination".)