Like some committees (5)
'like some committees' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ad hoc that I've seen before include "For one particular purpose" , "... without preparation" , "specific occasion" , "Intended for one specific matter" , "special operation" .)
(Another definition for adhoc that I've seen is " for this special purpose".)