Overlooked socialist sleeping (4,3)
I believe the answer is:
left out
'overlooked' is the definition.
The answer and definition can be both related to thinking as well as being past participle verbs.
Maybe there's a link between them I don't understand?
'socialist sleeping' is the wordplay.
'socialist' becomes 'left' (associated in meaning).
'sleeping' becomes 'out' (I've seen this before).
'left'+'out'='LEFT OUT'
Can you help me to learn more?
(Another definition for left out that I've seen is " Omitted".)