भीत्रार्थानां भयहेतुः

Definition saṃjñā कारकसंज्ञाDefinition saṃjñā अपादानसंज्ञा

In case of words implying 'fear' and 'protection from danger' that from which the danger of fear proceeds is called अपादान कारक.

Word by word (padaccheda)

Of - त्रा- अर्थानाम् ṣaṣṭhī · bahuvacanaTerm / subject भयहेतुः prathamā · ekavacana

Of Ṣaṣṭhī (6th case): "of X". In this kind of sūtra it marks a relation, not a substitution.
Term / subject Prathamā (1st case): the term being defined, or the subject of the statement.

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Full reading

-त्रा-अर्थानाम् भयहेतुः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

In case of words implying 'fear' and 'protection from danger' that from which the danger of fear proceeds is called अपादान कारक।
The verbs signifying 'fear' or 'protection' govern the object feared of, or the danger to be protected from, in the ablative case. Thus चौरेभ्यो बिभेति he is afraid of thieves, चौरेभ्य उद्विजते he is agitated because of the thieves. चौरेभ्यस्त्रायते or रक्षति. He protects or saves from the thieves.

Why do we say 'the cause of fear (bhaya-hetu) is put in the ablative case'? Observe अरण्ये बिभेति or त्रायते he fears or protects in the forest.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.