अत्र in this sacrifice, विद्वांस: learned men, ब्रह्मराक्षसा: Brahmarakshasas (celestial beings turned into demons), छिद्रम् flaw, मृगयन्ते हि will be looking out for, निहतस्य struck down यज्ञस्य of sacrifice, कर्ता च the doer, सद्य: immediately, विनश्यति will be destroyed.

The learned brahmarakshasas always look out for flaws in the sacrifice so that they can spoil it. If a flaw finds place, the doer of the sacrifice is immediately destroyed.