Everyone turns up? Not at all — that's plain (5)
I believe the answer is:
'that's plain' is the definition.
(a grassy plain)
'everyone turns up? not at all' is the wordplay.
'everyone' becomes 'all' (all people).
'turns up?' shows that the letters should be reversed in order (I've seen 'turning up' mean this).
'not at all' becomes 'no'.
'all' in reverse letter order is 'lla'.
(Other definitions for llano that I've seen before include "Treeless area" , "(South American) grassy plain" , "Flat area" , "Treeless grassy plain in South America" , "Grassy plain in South?America" .)