Assuming your flight from Chengdu lands on time, you should be able to clear the airport by 5:30-6pm. As the others have suggested, take the airport express into Central. From there you can grab something to eat or you could head up to the Peak to take in a view of HK Island, the harbour and over to Kowloon. If you were visiting during warmer weather, I would have recommended the Peak Cafe for its alfresco dining and views over Aberdeen (south side of HK island). There are lots of cafe/eateries in the shopping mall underneath the observation deck.

I would grab a cab (the queues for the tram down can be quite long). A cab down to Central terminal fare should be HKD$60-70 (£5-6) depending on traffic. I would take the Star Ferry over to TST (HKD$2.40, ~25p). Use the top deck.

If you haven't eaten by now, take a short walk up to Knutsford Terrace behind the Miramar Centre on Nathan Road (the road that is perpendicular to the ferry terminal). There are a number of different cuisines up there.

Alternatively you can try Ocean Terminal near the Star Ferry Terminal in TST. My favourite there is BLT. They make a nice New York strip and creamy spinach.

If you fancy a drink, head up to Felix on the 20th floor of the Pennisula hotel. Drinks are in line with UK but it has nice views over HK.

Once you are done, head back to Central either via the ferry or MTR (two stops from TST station on the Tseun Wan line towards Central). From there, follow the signs to the Airport Express to get back to the airport.

Immigration at HKG is pretty swift so you shouldn't have any problems getting airside.

Hope that helps

Merry Christmas

Paul