First-class sort of letter (7)
I believe the answer is:
capital
I believe this is a double definition.
'first-class' is the first definition.
(thesaurus)
'sort of letter' is the second definition.
(I know that capital is a type of letter)
(Other definitions for capital that I've seen before include "Means" , "Wealth; main city" , "Most important city" , "Kabul or Harare?" , "Big money?" .)