Sutra:
58. SthAnapramANaththAlE ukAram avathararaNArththam.

Meaning:
The ukAram is used in place of the word eva (only, exactly) and therefore it has the meaning of avatAraNam (descent).

Sri PBA Swami's Sarartha Deepikai:
The meaning of ukAram has to be taken as the same as if the word eva had been used. In the Vedas it is in place of eva that ukAram is usually used ("tadEvAgnis tat vAyus tat sUryas tadu candramA:"). Thus by being used in place of eva the meaning of ukAram can be taken as avatAraNam.