Plan is doomed from the start, dead, outsiders leave (4)
I believe the answer is:
idea
'plan' is the definition.
(both can mean an intention)
'is doomed from the start dead outsiders leave' is the wordplay.
'from the start' says to take the initial letters.
'dead outsiders leave' becomes 'ea' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The first letters of 'is doomed' is 'id'.
'id'+'ea'='IDEA'
Can you help me to learn more?
(Other definitions for idea that I've seen before include "Inkling" , "Proposal" , "Fancy" , "Thought or notion (4)" , "Notion, image" .)