Grain darkens over good many years (8)
I believe the answer is:
smidgeon
'grain' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'darkens over good many years' is the wordplay.
'darkens' becomes 'dims' ('dim' can be a synonym of 'darken').
'over' says the letters should be written backwards.
'good' becomes 'g' (abbreviation).
'many years' becomes 'eon' (I've seen this in another clue).
'dims' backwards is 'smid'.
'smid'+'g'+'eon'='SMIDGEON'
Can you help me to learn more?
(Other definitions for smidgeon that I've seen before include "bit" , "Tiny amount" , "A little" , "Small amount" , "Very small part" .)