Assuming you want all the underlines on the same height, here two possibilities

### ulem package

```
% !TeX TS-program = xelatex

\documentclass{article}
\usepackage{fontspec}
\setmainfont[Script=Devanagari,Mapping=devanagarinumerals]{Shobhika}

\usepackage[normalem]{ulem}
\newlength{\mydepth}

% insert character with heighest depth here
\settodepth{\mydepth}{क्लू}

\renewcommand{\ULthickness}{0.15ex}
\setlength{\ULdepth}{\dimexpr\mydepth+0.3ex\relax}

\begin{document}
	
	\uline{अबकड} \uline{कू} \uline{क्लू} \uline{ट्टू}

\end{document}
```

![Screen Shot 2020-02-13 at 00.44.43.png](/image?hash=4fa6e596e2c0145a0cd8f73d507f251cf5abfcd9bb3ce7f403d4154db8b95f3c)

### classic underline

```
% !TeX TS-program = xelatex

\documentclass{article}
\usepackage{fontspec}
\setmainfont[Script=Devanagari,Mapping=devanagarinumerals]{Shobhika}

\newlength{\mydepth}
\newlength{\myheight}

% insert character with heighest depth here
\settodepth{\mydepth}{क्लू}
\settoheight{\myheight}{क्लू}

\newcommand{\fix}{\rule[-\mydepth]{0pt}{\dimexpr\mydepth+\myheight\relax}}
\let\oldunderline\underline
\renewcommand{\underline}[1]{\oldunderline{\fix #1}}

\begin{document}

	\underline{अबकड} \underline{कू} \underline{क्लू} \underline{ट्टू}

\end{document}
```

![Screen Shot 2020-02-13 at 00.47.07.png](/image?hash=694cf275653794b981c0769210669d7fcf1edad89488673272cae74c08eda9ce)