。
She's not only smart but also hardworking.
Literal
She [topic-は] head [subject-が] good only [not only-だけでなく] hardworking also is.
~だけでなく~も is the standard pattern for 'not only X but also Y.' Here 頭が良い ('smart') is the first quality, and 勤勉でもある ('is also hardworking') is the second. The でもある structure uses で (te-form of the copula だ) + も (also) + ある, a slightly formal way to add a second predicate — in casual speech you'd more likely hear 勤勉だし.