  A non-canonical scalar tachyon field is a viable candidate for dark energy
and has been found to be in good agreement with observational data. Background
data alone cannot completely rule out degeneracy between this model and others.
To further constrain the parameters, apart from the distance measurements, we
study perturbations in the tachyon scalar field and how they affect matter
clustering. We consider two tachyon potentials for this study, an inverse
square potential and an exponential potential. We study the evolution of the
gravitational potential, matter density contrast, and dark energy density
contrast, and compare them with the evolution in the $\Lambda CDM $ model.
Although perturbations in dark energy at sub-Hubble scales are negligible in
comparison with matter perturbations, they cannot be ignored at Hubble and
super-Hubble scales ($\lambda_p > 1000$ Mpc). We also study the evolution of
growth function and growth rate of matter, and find that the growth rate is
significantly suppressed in dark energy dominated era with respect to the
growth rate for $\Lambda CDM$ model. A comparison of these models with Redshift
Space Distortion growth rate data is presented by way of calculating
$f\sigma_8(z)$. There is a tension of $2.9\sigma$ ($2.26\sigma$ ) between
growth rate data and Planck-2015 (Planck-2018) Cosmic Microwave Background
Radiation data for $\Lambda CDM$ model. We present constraints on free
parameters of these models and show that perturbations in the tachyon scalar
field reduce this tension between different data sets.
