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The lawyer has solid evidence that she is innocent.
Literal
Lawyer [topic-は] she [subject-が] innocent [is-quotative-だという] strong evidence [obj-を] is-holding.
Two notable bits. (1) ~だという is the quotative complementizer 'that [clause]' when the clause contains a copular predicate — literally 'that is called X.' Here linking the proposition 'she is innocent' to 証拠 ('evidence'). (2) 証拠を握っている ('to hold evidence') extends 握る ('grasp') into the figurative 'have in one's possession.' The resulting state reading: 'is in possession of strong evidence.'