I think it'd be perfect if there's a scene when Clarke's tending a Grounder woman with an infant. The mom needs to rest, so she lets Clarke hold her baby. So we see Clarke singing to the baby, and then Bellamy enters and hovers by the door, watching in awe. The camera really focuses on his face as it dawns on him. "This is Clarke, this is how compassionate she is. She's mothered everyone this entire time and she's so loving." Thus the beginning of ~strong~ romantic feelings from our pal Bell.