My father was told by the doctor to cut down on cigarettes.
Literal
Father [topic-は] doctor [from-から] tobacco's amount [object-を] reduce [quotative-と] was-told.
減らせ is the imperative form of 減らす — the doctor ordered 'reduce it!' bluntly. The imperative is softened by being reported indirectly via と言われた (passive of 言う). This is how Japanese handles indirect commands: imperative inside quotation, framed with passive reporting.