Build up, as interest

ACCRUE

Think of how money or benefits gradually gather over time, often without immediate action—like rewards adding up in the background.

Build up, as interest" hints at the gradual increase of something over time, like money earning interest in a bank account. The word "accrue" fits perfectly because it means to accumulate or grow steadily, often used in financial contexts. There’s no wordplay here—it’s a straightforward definition where the clue directly describes the answer’s meaning.