John Gill Commentary 1 Chronicles 2:26

"And Jerahmeel had another wife, whose name was Atarah; she was the mother of Onam." — 1 Chronicles 2:26 (ASV)

Jerahmeel had also another wife, whose name was Atarah
Distinct from his wife before named; or "another woman", which is a phrase for an harlot or concubine, (Judges 11:2) which she might be, as Kimchi observes; though the former seems best:

she was the mother of Onam ;
and perhaps was the only son she bore to Jerahmeel, of whose sons see (1 Chronicles 2:28) .