It's not this empty threat (4)
I believe the answer is:
that
'it's not this' is the definition.
'empty threat' is the wordplay.
'empty' suggests removing the centre.
'threat' with its middle taken out is 'THAT'.
(Other definitions for that that I've seen before include "The (one) there" , "Just like --; -- is to say" , "Not 8 Ac but the other" , "See 5" , "Yon" .)