Four in six days will be bright (5)
I believe the answer is:
vivid
'bright' is the definition.
(I know that bright can be written as vivid)
'four in six days' is the wordplay.
'four' becomes 'IV' (Roman numerals).
'in' means one lot of letters goes inside another.
'six' becomes 'VI' (Roman numerals).
'days' becomes 'd' (abbreviation for days).
'iv' going within 'vi' is 'vivi'.
'vivi'+'d'='VIVID'
'will be' acts as a link.
(Other definitions for vivid that I've seen before include "Brightly coloured" , "Brilliant" , "Intensely striking in colour" , "Brilliantly coloured - full of life" , "Presenting a clear and striking picture" .)