In short, it's of graphic significance (5)
I believe the answer is:
photo
'graphic significance' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'in short it's of' is the wordplay.
'in short' becomes 'phot' (I can't explain this - if you can you should believe this answer much more).
'it's of' becomes 'o'.
'phot'+'o'='PHOTO'
Can you help me to learn more?
(Other definitions for photo that I've seen before include "Snap, exposure, in short" , "Picture, in short" , "Snap, pic" , "Camera picture" , "Snap, picture" .)