。
There's always a deeper layer to deception.
Literal
Behind [at-には] behind [subject-が] exists.
裏には裏がある — literally 'behind the back, there is a back.' A proverb-like expression meaning that behind every trick or scheme, there's another one lurking. The double use of 裏 ('reverse side, hidden side') creates a compact, memorable structure. This is the kind of sentence Japanese excels at — a pithy, philosophical observation in minimal words.