Aunt crosses a river, lake and plain (2,7)
I believe the answer is:
au naturel
'plain' is the definition.
(I've seen this before)
'aunt crosses a river lake' is the wordplay.
'crosses' indicates putting letters inside.
'river' becomes 'ure' (river in Yorkshire).
'lake' becomes 'l' (geographical abbreviation).
'aunt' placed around 'a' is 'aunat'.
'aunat'+'ure'+'l'='AU NATUREL'
'and' is the link.
(Other definitions for au naturel that I've seen before include "served without dressing" , "Very simply" , "Naked in French, to aural tune" , "In the nude - in France" , "Cooked plainly - naked" .)