I know that
div {
  a {
    ...
  }
}
in LESS will stand for
div a {
  ...
}
in CSS. The question is what is the LESS code for this:
div > a {
  ...
}
?
You can simply use
div {
  > a{
    color: blue;
  }
}
or
div {
  & > a{ /* & represents the current selector parent, so will be replaced with div in this case in the output */
    color: blue;
  }
}
Both have the same effect and would produce the following as compiled output.
div > a {
  color: blue;
}
Note: You can follow the same approach for the + (adjacent sibling combinator) and the ~ (general sibling combinator) also. 
