Fruitful source — a very quiet let, really excellent earnings initially (5,4)
I believe the answer is:
apple tree
'fruitful source' is the definition.
'apple tree' can be an answer for 'source' (I have seen 'source of fruit' mean 'apple tree' so perhaps 'source' could also mean 'apple tree'). I'm not certain of the 'fruitful' bit.
'a very quiet let really excellent earnings initially' is the wordplay.
'very quiet' becomes 'pp'.
'initially' indicates taking the first letters.
The first letters of 'really excellent earnings' is 'ree'.
'a'+'pp'+'let'+'ree'='APPLE-TREE'
Can you help me to learn more?
(Other definitions for apple tree that I've seen before include "Source of the growth that inspired Newton?" , "where Jonathan hangs out" , "fruitful growth" , "may be source of windfall" , "source of fruit" .)