Readily understood by many a listener (5)
I believe the answer is:
clear
'readily understood' is the definition.
'clear' can be an answer for 'understood' (is that clear?). I am not sure about the 'readily' bit.
'many a listener' is the wordplay.
'many a' becomes 'cl' (I am not sure about this - if you are sure you should believe this answer much more).
'listener' becomes 'ear' (body part which hears or listens).
'cl'+'ear'='CLEAR'
'by' is the link.
Can you help me to learn more?
(Other definitions for clear that I've seen before include "Bright" , "Understandable" , "Free from blame" , "Transparent or cloudless" , "beyond doubt" .)