  We present a broadband spectral analysis of the atoll source 4U~1636-536
observed for $\sim92$ ks with \nustar{}. The source was found to be in a
low-luminosity state during this observation with $3-79 \kev{}$ X-ray
luminosity of $L_{3-79 keV}=(1.03\pm0.01)\times 10^{37}$ ergs/s, assuming a
distance of 6 kpc. We have identified and removed twelve type-I X-ray bursts
during this observation to study the persistent emission. The continuum is well
described by a thermal Comptonization model {\tt nthcomp} with $\Gamma\sim1.9$,
$kT_{e}\sim28 \kev{}$, and $kT_{s}\sim0.9\kev{}$. The \nustar{} data reveal a
clear signature of disc reflection, a significantly broad Fe-K emission line
(around $5-8\kev{}$), and the corresponding reflection hump (around
$15-30\kev{}$). We have modeled the data with two relativistically blurred
reflection models. Both families of reflection models prefer truncated inner
disc radii prior to the ISCO. We find that the inner disc is truncated with an
inner radius of $R_{in}=(3.2-4.7)\;R_{ISCO}$ ($\simeq16-24\,R_{g}\: \text{or}\:
36-54$ km). This inner disc radius suggests that the neutron star magnetic
field strength is $B\leq2.0\times10^{9}$ G.
