Is he naive but cold? (9)
I believe the answer is:
heartless
'cold?' is the definition.
(thesaurus)
'is he naive' is the wordplay.
'naive' becomes 'artless' (similar in meaning).
'he'+'artless'='HEARTLESS'
'but' acts as a link.
(Other definitions for heartless that I've seen before include "hard?" , "with no concern for others?" , "Unkind" , "Uncaring" , "Without pity or feeling, being defective in cardiac way" .)