For a special purpose (2,3)
'for a special 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 "not in general practice" , "(Created) for particular purpose" , "particular case" , "For one specific case" , "Applied to a particular situation" .)
(Another definition for adhoc that I've seen is " for this special purpose".)