One metal horseshoe coming up! (4)

I believe the answer is:

unit

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'one' is the definition.
(synonyms)

'metal horseshoe coming up' is the wordplay.
'metal' becomes 'U' (U is an example).
'horseshoe' becomes 'tin' (I can't explain this - if you can you should believe this answer much more).
'coming up' says the letters should be written in reverse.
'tin' backwards is 'nit'.
'u'+'nit'='UNIT'

Can you help me to learn more?

(Other definitions for unit that I've seen before include "Part of a kit" , "Single item, maybe part of a set" , "Standard for measuring" , "Subdivision of a military group" , "Single piece - kitchen cabinet" .)

I've seen this clue in the Evening Standard.

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