documentclass{letter}

signature{Maurice Moss}
newcommand{fillinadd}[1]{#1}

begin{document}

begin{letter}{
Fire Department \
fillinadd{DC}}

opening{Dear Sir / Madam,}
Fire! Fire! Help me! 123 Carrendon Road. Looking forward to hearing from you.
closing{All the best,}

end{letter}
end{document}