For one specific purpose (2,3)
I believe the answer is:
ad hoc
'for one specific purpose' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ad hoc that I've seen before include "Latin notation - for a specific purpose" , "(Created) for particular purpose" , "For this special purpose (Latin)" , "specific occasion" , "(Of committee) formed for a single purpose" .)