Agent involved in the preparations (3)
I believe the answer is:
rep
'agent' is the definition.
(short for representative)
'involved in the preparations' is the wordplay.
'involved in' indicates a hidden word.
'REP' is hidden in the letters of 'preparations'.
(Other definitions for rep that I've seen before include "Representative; fabric" , "Salesperson" , "Theatre; fabric; salesman" , "Sort of theatre" , "Fabric with a ribbed surface" .)