from OED online:

1. Mus. a. = CADENCE n. 4. b. (See quot. 1944.)

1740 in GRASSINEAU Mus. Dict. 1876 STAINER & BARRETT Dict. Mus. Terms 98/1 Clausula, a close or cadence, e.g.: clausula falsa, a false cadence; clausula finalis, a final cadence, &c. 1889 GROVE Dict. Mus. IV. 592/1 Clausula, the mediæval name for what is now called a Cadence, or Close. The most important Close employed in Polyphonic Music, is the Clausula vera, or True Cadence, terminating on the Final of the Mode. 1944 W. APEL Harvard Dict. Mus. 155/2 In the repertory of the School of Notre Dame (c. 1200)..clausula is a polyphonic composition using as a cantus firmus a short melisma of a chant.

2. The close or end of a period, esp. one in ancient or medieval Latin having a definable cadence. (Cf. CLAUSE 1b.)

1900 J. E. SANDYS in Class. Rev. XIV. 136/2 The favourite clausulae of late Greek Prose and of Cicero are also examined. 1906 A. SLOMAN Gram. Class. Latin III. 387 In Cicero's Orations at any rate the Clausula is regulated by definite metrical rules. 1957 N. FRYE Anat. Criticism IV. 264 Recurring units of rhythm like the clausulae of Cicero.