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 can't understand how they can define each other.
'eden largely maintained by masses' is the wordplay.
'eden' becomes 'utopia' (I've seen this in another clue).
'largely' means to remove the last letter (most but not all of the letters).
'maintained' means one lot of letters goes inside another (inserted letters get controlled or maintained).
'by masses' becomes 'alot' (I can't justify this - if you can you should believe this answer much more).
'utopia' with its last letter taken away is 'utopi'.
'utopi' placed 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" .)