"Not that!" (4)
(Other definitions for only that I've seen before include "Unique; merely" , "And nothing more; sole" , "Merely; single" , "Solely; except that" , "Simply" .)
'not that' is the definition.
(this one as opposed to that one)
This is the entire clue.
(Other definitions for this that I've seen before include "The thing here referred to" , "One particular" , "The thing just mentioned" , "near thing" , "Hits (anag.)" .)