South American city without a temperature scale (5)
I believe the answer is:
ratio
'scale' is the definition.
(thesaurus)
'south american city without a temperature' is the wordplay.
'south american city' becomes 'rio' (I've seen this before).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'temperature' becomes 't' (abbreviation).
'a'+'t'='at'
'rio' going around 'at' is 'RATIO'.
(Other definitions for ratio that I've seen before include "Mathematical relationship" , "Proportion, relation" , "A trio (anag)" , "2:1, for example" , "Expression of relationship" .)