First-rate, formal English (5)
I believe the answer is:
prime
'first-rate' is the definition.
(thesaurus)
'formal english' is the wordplay.
'formal' becomes 'prim' (I've seen this in other clues).
'english' becomes 'e' (abbreviation).
'prim'+'e'='PRIME'
(Other definitions for prime that I've seen before include "First-rate; type of number" , "Chief or most important, like a minister" , "Fundamental - of highest quality" , "Of first quality" , "Maybe eleven" .)