Let C be a 2-connected projective curve either reduced with planar singularities or contained in a smooth algebraic surface and let S be a subcanonical cluster (i.e. a 0-dimensional scheme such that the space H 0 (C, I S K C ) contains a generically invertible section). Under some general assumptions on S or C we show that h 0 (C, I S K C ) ≤ p a (C) − 1 2 deg(S) and if equality holds then either S is trivial, or C is honestly hyperelliptic or 3-disconnected.As a corollary we give a generalization of Clifford's theorem for reduced curves with planar singularities.