In a PC environment I have a virtual keypad interface for phone dialing purposes. I need both a clear character function and a clear field function. Is it OK to append both functions to a single delete button where clicking the button once deletes a character and clicking and holding clears the field?
I think you could run into problems with this interface as you propose it, that is, by binding two different events to the same interface button.
Both double clicks and click and hold can be awkward for any user who does not have good control of their hands (age, illness etc.). They may well end up accidentally clearing the entire field. Considering such a user is more likely to make mistakes entering information in the first place this could lead to an awful user experience for such users.
Looking at other options, I see no problem in having two delete buttons, as long they are clearly labelled (not like the calculators infuriating C and CE buttons), neither do I see too many problems with a right click and clear field being one of the options available there. Right click for more options is a common pattern than click and hold.
I would look at your users habits and try to analyse how they behave using your interface. Firstly, look at how often they actually want to clear the entire field, this might help decide whether to relegate this function to a less obvious menu and away from your primary interaction methods. Whether you go for double click, or click and hold, you could also measure the usage and decide on a sensible default, as well as making the length of time to hold (or between clicks) adjustable.
Long click is not a standard interaction with a mouse, unlike a tap-and-hold on a touch-screen interface. There isn't a browser event for it, for example: you just get "Click" and would have to do some fancy work with "MouseDown" and "MouseUp" to work out whether it was a short click or a long one.
Why not use the standard double click functionality? Click and DoubleClick are separate identifiable events.