  We study the neutralino radiative decay into the lightest supersymmetric
particle (LSP) in the framework of a minimal anomalous U(1)' extension of the
MSSM. It turns out that in a suitable decoupling limit the axino, which is
present in the St\"uckelberg multiplet, is the LSP. We compute the branching
ratio (BR) for the decay of a neutralino into an axino and a photon. We find
that in a wide region of the parameter space, the BR is higher than 93% in
contrast with the typical value (<=1%) in the CMSSM.
