Twenty per cent of half if three is required (5)
I believe the answer is:
fifth
'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" .)