Formed for a single purpose (2,3)
I believe the answer is:
ad hoc
'formed for a single purpose' 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 committee) formed for a single purpose" , "(Created) for particular purpose" , "as required" , "Done for a particular purpose only" , "particular case" .)