अत्र in this sacrifice, विद्वांस: learned men, ब्रह्मराक्षसा: brahma rakshasas(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 strike into it. If the sacrifice is damaged, the doer of the sacrifice will be immediately destroyed.