I saw in some football games ，when the second extra time ended，the subtitle showed ＂end of the second extra＂
in ＂second extra＂ ，is＂extra＂a noun？ or ＂extra＂is adjective，and they omit ＂time ＂ for convenience
It's a noun in that case. As an American, I'm unfamiliar with the soccer term, but in baseball, we similarly say extra or extras to refer to extra innings. (American football would say overtime, e.g., "first overtime," or "overtime one.")
'Extra time' - 'extra' remains an adjective describing 'time'.
In 'second extra' - 'time' has been omitted for convenience and because all football fans would know that 'extra' actually means 'extra time'. 'extra' briefly becomes a noun, with 'second' being the adjective.