The gateman has become a tycoon (7)

I believe the answer is:

magnate

I'm a little stuck... Click here to teach me more about this clue!

'tycoon' is the definition.
(I know that tycoon can be written as magnate)

'the gateman has become a' is the wordplay.
'gateman' anagrams to 'MAGNATE'.
But, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for magnate that I've seen before include "Wealthy or powerful businessman" , "Tycoon" , "Important fellow" , "Fat cat" , "Fellow with a lot of money" .)

I've seen this clue in The Irish Independent.

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