Ecco qua
`
\documentclass{elsart}
\usepackage[square,numbers]{natbib}
\usepackage{hyperref}
\usepackage[T1]{fontenc}
\usepackage{amsmath,onlyamsmath,amsfonts,graphicx,latexsym,enumerate,enumitem,epsfig,
booktabs,lineno}
\makeatletter
\def\keyword{%
\global\@haskeywordstrue % Implies rules are to be printed
\hyphenpenalty\sv@hyphenpenalty % restore \hyphenpenalty
\def\sep{\unskip, } % separator for multiple keywords
\def\MSC{\par\leavevmode\hbox{%
\textit{2000 MSC (2000 Mathematical subject classification):\ }}}
\def\PACS{\par\leavevmode\hbox {\it PACS:\ }}%
\global\setbox\t@keyword=\vbox\bgroup
\@keywordsize
\parskip\z@
\vskip 10\p@ \@plus 2\p@ \@minus 2\p@ % One line of space above keywords.
\noindent\@keywordheading
\justify@off % Keywords are not justified.
\ignorespaces}
\makeatother
\begin{document}
\begin{frontmatter}
testo
\begin{keyword}
\MSC
\end{keyword}
\end{frontmatter}
\end{document}`
Se avessi messo un esempio compilabile avresti fatto prima.
Ciao
Andrea