उषासोषसः

Operational rule vidhi

For उषस् is substituted उषासा in a देवता - द्वन्द समास.

Word by word (padaccheda)

Substitute / term उषासा prathamā · ekavacanaTarget उषसः ṣaṣṭhī · ekavacana

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

उषासा उषसः

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)

उषसः → उषासा

Replace uṣasaḥ with uṣāsā. Inherited words (anuvṛtti) may add further conditions.

For उषस् is substituted उषासा in a देवता - द्वन्द समास।
Thus:- उषासासूर्यम्, उषासानक्ता ॥

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