Plain is one, in degrees Celsius (5)
I believe the answer is:
basic
'plain' is the definition.
(thesaurus)
'one in degrees celsius' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'degrees' becomes 'bas' (BA is an example).
'celsius' becomes 'c' ('c' can be a synonym of 'celsius').
'bas'+'c'='basc'
'i' going into 'basc' is 'BASIC'.
'is' acts as a link.
(Other definitions for basic that I've seen before include "Rudimentary - programming language" , "Elementary - without extras" , "Plain" , "Popular programming language" , "Spartan" .)