Insect on cacti circling round, right where it's very cold (9)
I believe the answer is:
antarctic
'where it's very cold' is the definition.
'insect on cacti circling round right' is the wordplay.
'insect' becomes 'ant' (ant is a kind of insect**).
'on' says to put letters next to each other.
'circling' indicates anagramming the letters.
'round' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'cacti' is an anagram of 'actic'.
'actic' going around 'r' is 'arctic'.
'ant'+'arctic'='ANTARCTIC'
(Other definitions for antarctic that I've seen before include "Relating to south polar region" , "Penguin's home" , "near the south pole" , "Relating to southern polar region" , "Area around the South Pole" .)