The code
\documentclass{book}
\usepackage{imakeidx}
\let\cleardoublepage\clearpage
\makeindex
\usepackage[itemlayout=singlepar]{idxlayout}
\begin{document}
\LARGE
This is the first paragraph.\index{AA! first}\index{AA! second}\index{AA! third}
This is the second paragraph.\index{AAB! an entry}
This is the third paragraph.\index{AAC! another entry}
\idxlayout{columns=1}
\printindex
\end{document}
produces the index:
How may I specify ahead of time the gap (say, 5pt, 10pt, 12pt, etc.) between AA and AAB; between AAB and AAC ?