Can I defy a sacrifice?

Yes, you can Defy a Sacrifice once it is on the chain.

However, it is important to understand the timing of when this happens. According to Rule 357 and RiftJudge FAQ #10480, the additional cost of a spell (in this case, killing a friendly Mighty unit) is paid instantly when you play the spell, before the spell is finalized and added to the chain.

The Sequence:

1. Announcement: The player announces they are playing Sacrifice.
2. Payment of Costs: The player kills a friendly Mighty unit (5+ might) as the additional cost. This unit is sent to the trash immediately.
3. Finalization: With the cost paid, Sacrifice is finalized and placed on the chain.
4. Reaction: You (the opponent) receive priority and play Defy to counter Sacrifice.

The Result:

Because the unit was already killed as a cost to play the spell, it is already in the trash by the time Defy is even played. Rule 425.1.c states that countering a card does not refund any costs paid to play it. Therefore, Sacrifice is countered and cleared from the chain, but the unit sacrificed to play it remains in the trash and does not return to play.