Reliably hot region also coldest irregularly (5,3,3)
I believe the answer is:
costa del sol
'reliably hot region' is the definition.
'costa del sol' can be an answer for 'region' (I have seen 'Spanish region' mean 'costa del sol' so perhaps 'region' could also mean 'costa del sol'). I'm unsure of the 'reliably hot' bit.
'also coldest irregularly' is the wordplay.
'irregularly' is an anagram indicator.
'also'+'coldest'='alsocoldest'
'alsocoldest' is an anagram of 'COSTA DEL SOL'.
Can you help me to learn more?
(Other definitions for costa del sol that I've seen before include "Spanish beach" , "In place of holiday" , "Region around Malaga" , "criminals' haven?" , "Spanish resort region" .)