  In systems of diffusing particles, we investigate large deviations of a
time-averaged measure of clustering around one particle. We focus on biased
ensembles of trajectories, which realise large-deviation events. The bias acts
on a single particle, but elicits a response that spans the whole system.
  We analyse this effect through the lens of Macroscopic Fluctuation Theory,
focussing on the coupling of the bias to hydrodynamic modes. This explains that
the dynamical free energy has non-trivial scaling relationships with the system
size, in 1 and 2 spatial dimensions.
  We show that the long-ranged response to a bias on one particle also has
consequences when biasing two particles.
