Some guidelines...
- Don't wrap your math components in braces;
{x} removes the appropriate spacing around x.
- Don't use
{\rm ..} for roman text font. Rather use \text{..} as supplied by amsmath.
\in denotes "member of", as opposed to \epsilon.
- If you're using proper operators, you can underset an argument using "limits".

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\DeclareMathOperator*{\argmin}{arg\,min}
\begin{document}
\begin{align}
\label{Eq:argmin}
& x = \argmin_p \{N_{\text{clip}}(\bar{\mathbf{x}})\} \\
& \text{subject to } p \in \Theta \nonumber
\end{align}
\end{document}
For better/different definitions of \argmin, see Command for argmin or argmax?
1: https://i.stack.imgur.com/W7jhG.pngSome guidelines...
- Don't wrap your math components in braces;
{x} removes the appropriate spacing around x.
- Don't use
{\rm ..} for roman text font. Rather use \text{..} as supplied by amsmath.
\in denotes "member of", as opposed to \epsilon.
- If you're using proper operators, you can underset an argument using "limits".

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\DeclareMathOperator*{\argmin}{arg\,min}
\begin{document}
\begin{align}
\label{Eq:argmin}
& x = \argmin_p \{N_{\text{clip}}(\bar{\mathbf{x}})\} \\
& \text{subject to } p \in \Theta \nonumber
\end{align}
\end{document}
For better/different definitions of \argmin, see Command for argmin or argmax?
\mininstead of{\rm min}... – karlkoeller Sep 26 '13 at 16:25