Region's average temperature (4)
I believe the answer is:
part
'region's' is the definition.
('part' can be a synonym of 'region')
'average temperature' is the wordplay.
'average' becomes 'par' (I've seen this before).
'temperature' becomes 't' (abbreviation).
'par'+'t'='PART'
(Other definitions for part that I've seen before include "Piece; divide" , "Sever; portion" , "Constituent - something to play?" , "Section of a whole" , "unemployed actor hopes for" .)