Picture representing key field essential to agriculture (7)
I believe the answer is:
tableau
'picture' is the definition.
(a tableau is a picture or display)
'key field essential to agriculture' is the wordplay.
'key' becomes 'tab' (key on a computer keyboard).
'field' becomes 'lea' (lea can mean a meadow or field of grass).
'essential to' indicates the central letters.
The central letter of 'agriculture' is 'u'.
'tab'+'lea'+'u'='TABLEAU'
'representing' acts as a link.
(Other definitions for tableau that I've seen before include "Group of people arranged as for a painting say" , "A scene with silent motionless group of people" , "A dramatic or picturesque scene, may be still" , "Diorama; vignette" , "Figures making a scene" .)