I do this regularly with a 7.5 hour transit in SIN. Arrive and go straight to the lounge (BP already issued at point of origin). Freshen-up, grab a drink (or two ), use the WiFi to contact locals and make final arrangements regarding time and place for dinner/drinks. Leave the lounge and out trough immigration.

On arrival back at the airport, back through immigration, comment about a quick dinner accounting for my 3-hour "visit" to Singapore, then back to the lounge for a shower, nightcap etc before boarding next leg.

This is why I choose SIN for my outbounds to Europe on QF/AY. Makes the long transit easy and fun.