अश्वाघस्यात्

Operational rule vidhi

In the छन्दस् (Vedas) long आ is substituted for the final of 1. अश्व and 2. अघ before the Denominative क्यच् .

Word by word (padaccheda)

Target अश्वाघस्य ṣaṣṭhī · ekavacanaSubstitute / term आत् prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

अश्वाघस्य आत्

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

As a rewrite rule (sketch from case endings)

अश्वाघस्य → आत्

Replace aśvāghasya with āt. Inherited words (anuvṛtti) may add further conditions.

Commentary

In the छन्दस् (Vedas) long आ is substituted for the final of 1. अश्व and 2. अघ before the Denominative क्यच् ।
As अश्वायन्तो मघवन् (Rig VII.32.23), मा त्वा वृका अघायवो विदन् ॥ This also indicates that other words do not lengthen their vowel in the Veda before क्यच्, as taught in 7.4.35. See Vartika to 3.1.8. The word अघायु occurs in Rig I.120.7.27; 3.

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