亻 is the person radical 人 in left-side form. It tags 偿 as a human action, grouping it with 借 (borrow), 付 (pay), and 价 (price) — repayment is something one person owes and gives back to another.
尝 supplies the reading cháng directly. The taste-and-try sense also flavors the meaning: to 偿 a debt is to give the other party a taste of what they're owed, to make good on a long-tasted obligation.
