For a particular purpose only (2,3)
I believe the answer is:
ad hoc
'for a particular purpose only' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ad hoc that I've seen before include "(Of group) formed for one purpose only" , "For a specific purpose - Latin expression" , "For a specific purpose, say an arrangement" , "specific occasion" , "(Of committee) formed for a single purpose" .)