Describe, describing hot old region (6)
I believe the answer is:
thrace
'old region' is the definition.
(historical term for part of southeastern Europe)
'describe describing hot' is the wordplay.
'describe' becomes 'trace' ('trace' can be a synonym of 'describe').
'describing' indicates putting letters inside (describe can mean to form the outline of).
'hot' becomes 'h'.
'trace' placed around 'h' is 'THRACE'.
(Other definitions for thrace that I've seen before include "Balkan area" , "old country" , "old part of Europe" , "The car (anag.); historical region" , "Ancient region now in Greece and Bulgaria" .)