What does the cryptic crossword clue "Intelligence services see note that's taken from a dupe (6)" mean?

The wordplay uses a charade (building blocks). MI (synonym="as in MI5, MI6") + C (synonym="see") + KEY (synonym="KEY") = MICKEY The answer is MICKEY.

Intelligence services see note that's taken from a dupe (6)

Guardian Cryptic #26070 | 21 Down | 2013-10-04 flower