Marxist definition: class struggle, abolition of capitalist property, collective ownership, systemic change.
Starmer’s record: social-democrat, pragmatic, incremental reforms, accepts market economy.
Why he's not Marxist: avoids ideological rigidity, no revolutionary agenda, shifted toward center-left, compromises for governance.
I suggest reading the Communist Manifesto, so you know what to argue against if nothing else. It's, y'know, free.
Good for you.
However many of us don't believe he has actually changed in his views since his days editing, and writing articles for Socialist Alternatives.