“起辩护作用的”可以翻译为“defensive in function/role” 或者 “serving a defensive purpose” 。
例如：
This statement is defensive in function, aiming to protect the accused.（这句话起辩护作用，旨在保护被告 。）
The evidence presented was serving a defensive purpose.（所呈现的证据起辩护作用 。）