Hi,

SQ usually use terminal 3 on their long-haul flight, but flight within SEA region either departing from terminal 2 or 3.
If you are lucky, you connecting flight to DPS would be on the same terminal and 1 hour should be enough. However, if it's on different terminal that would be tight.

I'm sure there will be staff on standby at your arrival gate to assist with tight connecting pax (they usually put sign up up with flight + route, and have buggy stnby to quickly transport pax)
Also, security check is at the departing gate at SIN, so it would be a lot quicker than other airports