नित्यं क्रीडाजीविकयोः

Operational rule vidhi

A word ending with a sixth case-affix is invariably compounded with a word ending with अक , when these affixes denote a sport or livelihood; and the compound is तत्-पुरुष.

Word by word (padaccheda)

Substitute / term नित्यम् prathamā · ekavacanaRight context क्रीडा- जीविकयोः saptamī · dvivacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

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Full reading

नित्यम् क्रीडा-जीविकयोः

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

Commentary

A word ending with a sixth case-affix is invariably compounded with a word ending with अक , when these affixes denote a sport or livelihood; and the compound is तत्-पुरुष।
This is a Nitya-samasa incapable of analysis. This anuvritti of the word 'na' which began with sútra 10, does not extend further. The affix तृच् never has the sense of sporting or livelihood; the only examples possible are of 'aka.' Thus उद्दालकपुष्पभजिका 'a sort of game played by the people in the eastern districts in which uddalaka flowers are broken or crushed.' So also वारणपुष्पप्रचायिका 'a play of gathering flowers.'

So also दन्तलेखकः 'one who earns his bread by painting or marking the teeth.' नखलेखक 'a nail-painter by profession.'

Why do we say 'when meaning sporting or livelihood.' Observe ओदनस्य भोजकः.

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