Aristocratic photographer on main road up mountainous area (9)
I believe the answer is:
snowdonia
'mountainous area' is the definition.
(I've seen this before)
'aristocratic photographer on main road up' is the wordplay.
'aristocratic photographer' becomes 'snowdon' (Lord Snowdon, 1930-2017).
'on' is a charade indicator (letters next to each other).
'main road' becomes 'AI' (resembles A1, major UK road).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'ai' written backwards gives 'ia'.
'snowdon'+'ia'='SNOWDONIA'
(Other definitions for snowdonia that I've seen before include "alpine region" , "somewhere in 10" , "Mountainous Welsh region" , "mountains" , "Win a snood in the Welsh county" .)