Immediately make a poor performance outside the theatre (6)
I believe the answer is:
pronto
'immediately' is the definition.
(pronto can mean at once or quickly)
'a poor performance outside the theatre' is the wordplay.
'performance' indicates anagramming the letters.
'outside' means one lot of letters goes inside another (some letters go outside others).
'the theatre' becomes 'NT' (National Theatre).
'poor' is an anagram of 'proo'.
'proo' placed 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" .)