Single frightfully hot tongue (7)
I believe the answer is:
english
'tongue' is the definition.
(tongue can mean a language)
'single frightfully hot' is the wordplay.
'frightfully' is an anagram indicator.
'hot' becomes 'h' (common abbreviation eg on taps).
'single' is an anagram of 'englis'.
'englis'+'h'='ENGLISH'
(Other definitions for english that I've seen before include "Race" , "Language spoken in his glen, strangely" , "An official language of the Republic of India" , "speech" , "to write these answers in?" .)