एकादिश्चैकस्य चादुक्

Operational rule vidhi

The Negative Particle remains unchanged in a word which begins with एक and of this एक there is the augment अदुक् (अद्).

Word by word (padaccheda)

Substitute / term एकादिः prathamā · ekavacanaIndeclinable च avyayaTarget एकस्य ṣaṣṭhī · ekavacanaIndeclinable च avyayaSubstitute / term आदुक् prathamā · ekavacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

Full reading

एकादिः च एकस्य च आदुक्

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)

एकस्य → एकादिः आदुक्

Replace ekasya with ekādiḥ āduk. Inherited words (anuvṛtti) may add further conditions.

Commentary

The Negative Particle remains unchanged in a word which begins with एक and of this एक there is the augment अदुक् (अद्)।
Thus एकेन न विंशतिः = एकान्नविंशतिः, एकान्नत्रिंशत् ॥ These are Instrumental compounds. The augment आदुक् is added to the final of the first member, so that we have optionally two forms एकात् न विंशतिः and एकान् न विंशति by 8.4.45.

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