Picture that's about modification of agriculture (5)
I believe the answer is:
image
'picture' is the definition.
(I know that picture can be written as image)
'that's about modification of agriculture' is the wordplay.
'that's' becomes 'ie' (abbreviation for 'id est', 'that is' in Latin).
'about' is an insertion indicator.
'modification of agriculture' becomes 'mag' (I can't explain this - if you can you should believe this answer much more).
'ie' enclosing 'mag' is 'IMAGE'.
Can you help me to learn more?
(Other definitions for image that I've seen before include "Visual or mental representation" , "Likeness, might be spitting" , "Visualisation" , "Picture (involving one letter change from 10)" , "Purplish pigment; pool" .)