Answer
$7\cdot x^5$
Work Step by Step
According to the Binomial Theorem we can obtain the $r+1$th term of the expansion of the binomial $(x+y)^n$ by the formula $_nC_rx^{n-r}y^r$.
Hence here it is: (by plugging in $n=8,r=3$ and $x,\frac{1}{2}$: $_{8}C_3(x^3)^{8-3}(\frac{1}{2})^3=56\cdot x^5\cdot \frac{1}{8}=7\cdot x^5$