Solver mentioned 3 not hard: that's for a timber source (3,4)
I believe the answer is:
yew tree
'not hard that's for a timber source' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'solver mentioned 3' is the wordplay.
'solver' becomes 'you' (the person solving the crossword).
'mentioned' shows a homophone (sound like).
'e' becomes 'tree' (I can't explain this - if you can you should believe this answer much more).
'you' is a homophone of 'yew'.
'yew'+'tree'='YEW-TREE'
Can you help me to learn more?
(Other definitions for yew tree that I've seen before include "Emblem of grief seen in graveyards" , "Conifer" , "Plant" , "One gave us bows" , "Timber supplier" .)