Glut of information, the source of which agent revealed externally (9)
I believe the answer is:
repletion
'glut of information the source of which agent revealed externally' is the definition.
'repletion' can be an answer for 'glut' (thesaurus). I'm not certain of the rest of the definition.
'which agent revealed externally' is the wordplay.
'which' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'agent' becomes 'rep' (short for representative).
'revealed' becomes 'leton' (to let on about something is to reveal it).
'externally' indicates putting letters inside (some letters go on the outside of others).
'rep'+'leton'='repleton'
'i' inserted inside 'repleton' is 'REPLETION'.
Can you help me to learn more?
(Other definitions for repletion that I've seen before include "State of being satisfactorily full" , "State of having had enough to eat" .)