。
She's honest enough, I suppose, but can you really trust her?
Literal
She [topic-は] sort-of honest [but-が] really trust-can [question-か]?
まあ here is a hedging discourse particle — closer to English 'well, you could say...' or 'sort of.' It softens the following adjective, framing 正直 ('honest') as a grudging concession rather than a firm endorsement. The clause-final が works as 'but,' setting up a contrast between the concession and the real question. 信用できる is the potential form of 信用する ('to trust') — 'be able to trust.' The whole rhetorical move — 'sure, X, but really Y?' — is a classic way to voice doubt politely.