Crossword Genius

Twenty per cent of half if three is required (5)

Ross

I believe the answer is:

fifth

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'twenty per cent' is the definition.
I can't tell whether this definition defines the answer.

'of half if three is required' is the wordplay.
'half' indicates taking half.
'three' becomes 'th' (I can't justify this - if you can you should believe this answer much more).
'is required' says to put letters next to each other.
'of' halved is 'f'.
'f'+'if'+'th'='FIFTH'

Can you help me to learn more?

(Other definitions for fifth that I've seen before include "(Of a monarch) V" , "US amendment guaranteeing the right to silence" , "Fraction" , "20 per cent" , "- Column is a group working for the enemy" .)

I've seen this clue in The Globe and Mail.
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