Lam Chun See, at Good Morning Yesterday, asks a question based on a 1980s street directory: Why is Clementi labelled as Pasir Panjang?
As you can see from Chun See's scan (above) the area around Clementi Road and Ulu Pandan Road is labelled thus.
IceMoon, having read Chun See's post, comes up with two possible answers: electoral boundaries or postal districts.
However, he notes that there are issues with both theories.
How big is (or was) the Pasir Panjang vicinity and how was the area demarcated?