。
Clearly somebody's lying.
Literal
Clearly someone [subject-が] lie [object-を] is-telling.
誰か = 'someone' (indefinite). 嘘をついている — 嘘をつく ('to lie / tell a lie') in the ~ている progressive form, giving the ongoing state 'is lying.' The collocation 嘘をつく uses つく ('to attach/stick') in a specialized sense — a Japanese peculiarity for lying verbs.