There's one message for clearing hud but it clears everything ( weapon icons, timer etc... )
I see one way, sent hudmessage with 0.0 delay ( fadein,fadeout,hold,fx = 0.0 ) ( it will clear the screen ) then sent your hudmessage.
The delay between clear hudmessage and new hudmessage should be something 0.05 - 0.1 sec
It depends on the ping that client is having.
Thats how i made my costom menu color plugin.
You can get the code here:
https://forums.alliedmods.net/showthread.php?t=250788
Unfinished zsparkle mod and menu.zs ( open that with any txt editor )
Edit: there's no other proper way to clean hudmessages ... sadly.
Messing with that message ( hudreset or something was it called ) will be really painful since when you pick up the weapon for example ... that icon appears in right side of the screen and after hudreset message it will disappear and you can't make it appear back on its original state ( fade or color )