Can I use Sett's buff power when defending?

No, you cannot use Sett, Brawler's ability to spend a buff while you are defending.

According to established Riftbound rulings (RiftJudge FAQ #8543), Sett, Brawler's ability does not have the ACTION or REACTION keywords. Because it lacks these keywords, it is considered a "base speed" ability and is subject to strict timing restrictions:

• It must be your turn.
• There must be no chain present.
• You must be outside of a Showdown.

Because defending occurs during a Showdown (or involves being in a combat state), you cannot meet these requirements while defending. While Sett, Brawler will still gain a buff when he conquers (which happens when attacking or defending), you must wait until it is your turn and you are outside of a Showdown to spend that buff for the +4 Might bonus.