Available from version 1.6.0
called at the end of Parser::clearState()
Define function:
public static function onParserClearState( &$parser ) { ... }
Attach hook: In extension.json:
	"Hooks": {
		"ParserClearState": "MediaWiki\\Extension\\MyExtension\\Hooks::onParserClearState"
Called from: File(s): Parser.php
Interface: ParserClearStateHook.php

For more information about attaching hooks, see Manual:Hooks .
For examples of extensions using this hook, see Category:ParserClearState extensions.

Details edit

$parser: Parser object being cleared