Can you defy Mindsplitter ability?

No, you cannot use Defy on Mindsplitter's "When you play me" ability.

According to the rules and FAQs:

1. Defy can only counter spells: Defy’s text explicitly states it targets a "spell." The "When you play me" effect of Mindsplitter is a triggered ability, not a spell (Rule 425.1; RiftJudge FAQ #1137).
2. No Ability Counters: As confirmed in RiftJudge FAQ #1137, there is currently no "ability counter" in the game that can negate a triggered ability like Mindsplitter's.
3. Unit vs. Ability: You cannot respond to the act of playing the Mindsplitter unit itself because it enters the board and resolves immediately (RiftJudge FAQ #9017). While you can respond to the triggered ability once it enters the chain, Defy is invalid here because it only targets spells, not triggered abilities.