Very difficult without early PODs that make Singapore less populous/Chinese-dominant, or that Malay nationalism isn't as much of a driving force (no Japanese occupation?). Maybe something like "Communists take over Singapore and Malaya invades it" but that's kinda cheating.
It is also worth noting that OTL the 1948-1963 Federation of Malaya did not include Singapore, and Tunku Abdul Rahman only agreed to absorb Singapore along with Sarawak and Sabah, with what we now know as Eastern Malaysia obviously intended as a counterweight to maintain a Malay majority. If Sarawak/Sabah had refused for some reason or another, they wouldn't have taken in Singapore in the first place.