Smart being at opening of theatre in New York (5)
I believe the answer is:
natty
'smart' is the definition.
(thesaurus)
'at opening of theatre in new york' is the wordplay.
'opening of' says to take the initial letters.
'in' is an insertion indicator.
'new york' becomes 'ny'.
The initial letter of 'theatre' is 't'.
'at'+'t'='att'
'att' inserted into 'ny' is 'NATTY'.
'being' acts as a link.
(Other definitions for natty that I've seen before include "Stylishly dressed" , "Stylish" , "Flashily smart" , "Smartly dressed" , "Smart, fashionable (colloq.)" .)