  We prove a power saving upper bound for the sum of Fourier coefficients
$\rho_f(\cdot)$ of a fixed cubic metaplectic cusp form $f$ over primes. Our
result is the cubic analogue of a celebrated 1990 Theorem of Duke and Iwaniec,
and the cuspidal analogue of a Theorem due to the author and Radziwill for the
bias in cubic Gauss sums.
  The proof has two main inputs, both of independent interest. Firstly, we
prove a new large sieve estimate for a bilinear form whose kernel function is
$\rho_f(\cdot)$. The proof of the bilinear estimate uses a number field version
of circle method due to Browning and Vishe, Voronoi summation, and
Gauss-Ramanujan sums. Secondly, we use Voronoi summation and the cubic large
sieve of Heath-Brown to prove an estimate for a linear form involving
$\rho_f(\cdot)$. Our linear estimate overcomes a bottleneck occurring at level
of distribution $2/3$.
