अपदान्तस्य मूर्धन्यः

Heading adhikāra अपदान्तस्य मूर्धन्यः इत्यधिकारः

This is an अधिकार, which goes till the end of the पाद. For the स्थानी that does not occur at end of a पद, the closest 'मूर्धन्य' letter comes as the आदेश.

Word by word (padaccheda)

Of अपदान्तस्य ṣaṣṭhī · ekavacanaTerm / subject मूर्धन्यः 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.

Full reading

अपदान्तस्य मूर्धन्यः

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

Commentary

Up to the end of the पद , is throughout to be supplied the following - 'A cerebral letter substituted always in the room of …, when this letter does not stand at the end of a word'.

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