Barren open country hot after high temperature (5)
I believe the answer is:
heath
'barren open country' is the definition.
(I've seen this before)
'hot after high temperature' is the wordplay.
'hot' becomes 'h'.
'after' means one lot of letters go next to another.
'high temperature' becomes 'heat' ('heat' can be a synonym of 'high temperature').
'h' after 'heat' is 'HEATH'.
(Other definitions for heath that I've seen before include "Moor" , "Open land with acid soil" , "part of Hampstead" , "Ex-Tory PM" , "Area of open land with gorse etc" .)