Crossword Genius

A tonic in the club (4)

Ross

I believe the answer is:

iron

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'club' is the definition.
(type of golf club)

I cannot understand how the remainder of the clue works.

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(Other definitions for iron that I've seen before include "Smooth - metal" , "A magnet attracts it" , "Decrease" , "Metallic element, symbol Fe" , "great strength" .)

I've seen this clue in the Evening Standard.
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