Four in six days can be very bright (5)
I believe the answer is:
vivid
'be very bright' is the definition.
(I've seen this before)
'four in six days' is the wordplay.
'four' becomes 'iv' ('IV' can be a synonym of 'four').
'in' means one lot of letters goes inside another.
'six' becomes 'vi' ('vi' can be a synonym of 'six').
'days' becomes 'd'.
'iv' going into 'vi' is 'vivi'.
'vivi'+'d'='VIVID'
'can' is the link.
(Other definitions for vivid that I've seen before include "Intensely striking in colour" , "Graphic" , "Very bright and colourful" , "Intense" , "Fresh and bright" .)