  We study multi-wavelength variability of a bare Seyfert 1 galaxy Fairall~9
using \swift{} monitoring observations consisting of $165$ usable pointings
spanning nearly two years and covering six UV/optical bands and X-rays.
Fairall~9 is highly variable in all bands though the variability amplitude
decreases from X-ray to optical bands. The variations in the X-ray and
UV/optical bands are strongly correlated. Our reverberation mapping analysis
using the {\tt JAVALIN} tool shows that the variation in the UV/optical bands
lag behind the variations in the X-ray band by $\sim 2-10{\rm~days}$. These lag
measurements strongly suggest that the optical/UV variations are mainly caused
by variations in the X-rays, and the origin of most of the optical/UV emission
is X-ray reprocessing. The observed lags are found to vary as
$\tau\propto\lambda^{1.36\pm0.13}$, consistent with the prediction,
$\tau\propto\lambda^{4/3}$, for X-ray reprocessing in a standard accretion
disc. However, the predicted lags for an standard accretion disc with X-ray
reprocessing using black hole mass ($M_{BH} \sim 2.6\times10^{8}~M_{\odot}$)
estimated from the reverberation mapping of broad emission lines and accretion
rate relative to the Eddington rate ($\dot{m_E} =0.02$) are shorter than the
observed lags. These observations suggest that accretion disc in Fairall~9 is
larger than that predicted by the standard disc model, and confirm similar
findings in a few other Seyfert 1 galaxies such as NGC~5548.
