Fool involved with Christmas appeal (3)
I believe the answer is:
sap
'fool' is the definition.
(I know that fool can be written as sap)
'involved with christmas appeal' is the wordplay.
'involved with' indicates a hidden word.
'SAP' can be found hidden inside 'christmas appeal'.
(Other definitions for sap that I've seen before include "tunnel" , "Plant juice; undermine" , "Plant liquid or general vitality" , "It's plant juice, idiot" , "Weaken; tree fluid" .)