Straight from the horse's mouth... (possible spoilers)

[Sam Brown]

No: HPFGUIDX 1458

> Do you remember that clock that JKR made pains in GoF to tell us 
> about, the one which has a hand for each weasley and a space around 
> the face for "mortal peril?"  It seems to me that she wouldn't have 
> invented the clock and specifically told us about the "mortal 
peril" 
> bit unless one of the Weasleys was going to be there shortly.
> 
> Vicki

when i read that in GOF i had a vision of a scene in a future book 
that goes something like this...

(Harry is at the Weasley's)
Harry wondered where Ron could be. 'Where's Ron?' he asked Mrs 
Weasley.
'I don't know,' said Mrs Weasley. 'Look at our clock.'
Harry looked at the clock and was alarmed to see Ron's hand pointing 
to 'mortal peril'.

Of course the scene would be much better written than that, though!

Also, what would happen the hand on the clock if one of the weasley's 
did die? there's no 'time' that says death (like there are 'times' 
for work and school). would the hand of that family member just drop 
off?