If you can't find a clip for it you can put a small washer in there or even a piece of cardboard, anything that allows the spring to put pressure down on the clip and hold the needle down.
The carb rebuild kits come with them, so if nothing else you could buy a rebuild kit for about $15 and get everything you need for now and the future to go through the carb.
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....