As TAB cannot be displayed in Excel cell, instead of using CHAR(9), in order to indent the line, I recommend using spaces. By using REPT() function, it will be easier to control the indent width by specifying the number of spaces (e.g. 4).
=CONCATENATE(A1, CHAR(10), REPT(" ", 4), A2)
Alternatively, I usually use & for concatenation to simplify the formula.
=A1 & CHAR(10) & REPT(" ", 4) & A2
Lastly, you have to format the cell as "Wrap text" by CTRL-1 > Alignment
