Simply to meet the case (2,3)
I believe the answer is:
ad hoc
'case' is the definition.
(I have seen 'particular case' mean 'ad hoc' so perhaps 'case' could also mean 'ad hoc')
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ad hoc that I've seen before include "For a specific purpose - latin phrase" , "Done for a particular purpose only" , "not in general practice" , "particular case" , "Devised for one particular occasion" .)