Classy stage actor's first to fill English National Theatre (7)
I believe the answer is:
elegant
'classy' is the definition.
(classy can mean elegant or stylish)
'stage actor's first to fill english national theatre' is the wordplay.
'stage' becomes 'leg' (a leg is a stage of a competition).
'actor's first' becomes 'a' (1st letter of 'actor').
'to fill' is an insertion indicator.
'english' becomes 'e' (abbreviation).
'national theatre' becomes 'NT' (abbreviation).
'leg'+'a'='lega'
'e'+'nt'='ent'
'lega' inserted inside 'ent' is 'ELEGANT'.
(Other definitions for elegant that I've seen before include "Refined gracefulness" , "Chic; smart" , "Cleverly simple" , "Debonair, stylishly dressed" , "Neat leg? (anag)" .)