Fear dyes used here and there will lose intensity over time (4)
I believe the answer is:
fade
'fear dyes used here and there' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'lose intensity over time' is the wordplay.
'lose intensity' becomes 'fe' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over' means one lot of letters goes inside another.
'time' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'fe' placed around 'ad' is 'FADE'.
'will' acts as a link.
Can you help me to learn more?
(Other definitions for fade that I've seen before include "Wither away" , "Grow paler" , "Die away" , "Grow fainter" , "Grow faint and disappear" .)