• adharmeṇa — against religious principles
  Six against one is the adharma named. Even modern kṣatriya code condemned ganging up; the Bhāgavatam preserves the principle.
• dhārmikam — one who follows dharma
  Sāmba was the dhārmika; the six were the adhārmika. The moral grammar of the verse is exact: Ugrasena does not call Sāmba flawless, but he calls him dhārmika relative to those who bound him.
• mṛṣye — I am tolerating
  From mṛṣ, 'to bear with.' A kṣatriya virtue when in the service of family unity. Forbearance is the elder's gift — and the Kurus are about to refuse it.
• aikya - kāmyayā — with the desire for unity
  Aikya = oneness, unity. The motive for Ugrasena's restraint is named explicitly — keep the family one. The Kurus will reject the gift; the consequence at Kurukṣetra is foreshadowed.

A father whose son has been beaten by a gang of strangers does not strike back; he names the wrong and offers forbearance. King Ugrasena (through Balarāma's mouth) does exactly this — naming the adharma, then granting amnesty for the sake of family.