$$\displaystyle{}\partial_{m}(A)$$
This prints $\partial_{m}(A);$ in the notebook, despite the fact that \del is not a LaTeX command. If you use this property to make a symbol printable, make sure to declare it \emph{before} any other properties are declared, otherwise the notebook will not know how to display the symbol and produce an error message. Note that the property is attached to a pattern (\del{#} in this case) which matches the expression in which the replacement has to be made. If the pattern matches, the replacement will be done on the head symbol (\del in this case). A pattern \del without the argument wildcard # would only replace when \del occurs without any arguments (as in e.g. \del + A). These settings have no effect in the command-line version.