Immediately make a poor performance outside the theatre (6)
I believe the answer is:
pronto
'immediately' is the definition.
(thesaurus)
'a poor performance outside the theatre' is the wordplay.
'performance' indicates anagramming the letters.
'outside' means one lot of letters goes inside another.
'the theatre' becomes 'nt' (National Theatre).
'poor' is an anagram of 'proo'.
'proo' going around 'nt' is 'PRONTO'.
'make' is the link.
(Other definitions for pronto that I've seen before include "Quickly (sl.)" , "Quick as you can, amigo" , "Right now, amigo" , "Quickly, promptly" , "Quickly, amigo" .)