  We consider the dynamical evolution of small-scale substructure in clusters
within two extreme alternate scenarios for their possible origin: 1) the
accretion of groups (or small clusters) on quasi-radial orbits, and 2) the
merger of clusters of similar masses, followed by the decoupling of their dense
cores. Using simple analytical arguments and checking with numerical
computations, we show that objects are destroyed by the tidal field of the
global cluster potential if their mean density is small compared to the mean
cluster density within the radius of closest approach of the group or detached
core. Accreted groups and small clusters are thus tidally disrupted in one
cluster crossing. Since the cores of clusters are much denser than groups, they
are considerably more robust to tides, but the least massive are destroyed or
severely stripped by tides, while the others are brought to the cluster center
by dynamical friction (and subsequently merge) in less than one orbit. The
longest lived substructures are detached cores, roughly ten times less massive
than the cluster, starting in near-circular orbits beyond $1 \, h^{-1} \, \rm
Mpc$ from the cluster center.
