The main picture? (8)
I believe the answer is:
seascape
'the main picture?' is the definition.
'seascape' can be an answer for 'picture?' (seascape is a kind of picture). I am unsure of the 'the main' bit.
'main picture?' is the wordplay.
'main' becomes 'sea' (the main is an old name for the sea).
'picture?' becomes 'scape' (I can't justify this - if you can you should believe this answer much more).
'sea'+'scape'='SEASCAPE'
Can you help me to learn more?
(Other definitions for seascape that I've seen before include "Painting - pea cases (anag)" , "Marine view/picture" , "work of art" , "Maritime scene" , "Coastal or ocean view" .)