Mā here is negative, and in combination with illā establishes restriction (qaṣr). Dharafat, from the root dh-r-f, is a feminine singular perfect verb referring to the shedding or flowing of tears. ʿAynāki is the dual subject in the nominative, marked by alif.
The clause means “your eyes shed tears for no purpose but ...”. Restricting the apparent reason to one purpose intensifies the poet’s complaint, even though the claim is rhetorically charged.
ElementGrammatical role
MāNegative particle
DharafatFeminine singular perfect verb
ʿAynākiNominative dual subject
IllāElement of restriction