Being negative means nothing in the National Theatre (3)
I believe the answer is:
not
'being negative' is the definition.
(I've seen this in another clue)
'nothing in the national theatre' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'in the' means one lot of letters goes inside another.
'national theatre' becomes 'NT' (abbreviation).
'o' put inside 'nt' is 'NOT'.
'means' acts as a link.
(Other definitions for not that I've seen before include "In no way, never" , "Negation of what follows" , "Word of negation" , "logically, operation needed" , "A negative word" .)