Solipsist's preoccupation (4)
I believe the answer is:
self
'solipsist's preoccupation' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for self that I've seen before include "A person's awareness of their own identity" , "Will perhaps" , "one's identity" , "Will the author" , "Ego, identity" .)