For this purpose only (2,3)
I believe the answer is:
ad hoc
'for this purpose only' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ad hoc that I've seen before include "specific occasion" , "Latin notation - for a specific purpose" , "not in general practice" , "special operation" , "Applied to a particular situation" .)