Improvised or impromptu (2,3)
This clue is a double definition.
'improvised' is the first definition.
(I've seen this before)
'impromptu' is the second definition.
(I've seen this in another clue)
(Other definitions for ad hoc that I've seen before include "Applied to a particular situation" , "(Created) for particular purpose" , "not in general practice" , "For a specific purpose - Latin expression" , "(Of group) formed for one purpose only" .)
(Another definition for adhoc that I've seen is " for this special purpose".)