Thoughtlessness of Eden largely maintained by masses (9)
I believe the answer is:
autopilot
'thoughtlessness' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'eden largely maintained by masses' is the wordplay.
'eden' becomes 'utopia' (I've seen this before).
'largely' means to remove the last letter (most but not all of the letters).
'maintained' indicates putting letters inside (inserted letters get controlled or maintained).
'by masses' becomes 'alot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'utopia' with its final letter taken off is 'utopi'.
'utopi' inserted inside 'alot' is 'AUTOPILOT'.
'of' is the link.
Can you help me to learn more?
(Other definitions for autopilot that I've seen before include "requiring no help when stacking?" , "Airman's 'George'" , "Device for controlling an aircraft" , "It can fly plane" , "course controller" .)