Dolt is terribly unemotional (6)
I believe the answer is:
stolid
'unemotional' is the definition.
(similar in meaning)
'dolt is terribly' is the wordplay.
'terribly' is an anagram indicator.
'dolt'+'is'='doltis'
'doltis' anagrammed gives 'STOLID'.
(Other definitions for stolid that I've seen before include "Impassive or apathetic" , "Emotionally insensitive" , "Bovine" , "Unemotional - dense" , "Not excitable, doesn't show emotion" .)