documentclass{article}

usepackage[style=authortitle]{biblatex}
usepackage{filecontents}

begin{filecontents}{jobname.bib}
@book{mybook,
    author = {Author},
    title = {Book Title},
    date = {2018},
}

@customa{myentry,
    author = {Buthor},
    title = {My entry's title},
    date = {2017},
    xref = {book},
}

end{filecontents}

addbibresource{jobname.bib}

DeclareBibliographyDriver{customa}{%
    usebibmacro{bibindex}%
    usebibmacro{begentry}%
    printnames{author}%
    setunit{addcommaspace}%
    printfield{title}%
    newunitnewblock
    usebibmacro{in:}%
    entrydata*{mybook}{%
        printnames{author}%
        setunit{addcommaspace}%
        printfield{title}%
        newunitnewblock
        printtext{Which contains:space}%
        printnames{savedauthor}%
        setunit{addcommaspace}%
        printfield{savedtitle}%
        setunit{addcommaspace}%
        printdate
%       printsaveddate  % <- this doesn't work
        }%
    usebibmacro{finentry}%
}


begin{document}
nocite{*}
fullcite{myentry}

end{document}