Source almost discovered by end of August (5)
I believe the answer is:
fount
'source' is the definition.
(fount can mean a fountain or source)
'almost discovered by end of august' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'discovered' becomes 'found' (to discover is to find).
'by end of august' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'found' with its last letter taken away is 'foun'.
'foun'+'t'='FOUNT'
Can you help me to learn more?
(Another definition for fount that I've seen is " Source; set of type of one size".)