Become absorbed in thought
I believe the answer is:
muse
'become absorbed in thought' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for muse that I've seen before include "Inspiration for poet etc - ponder" , "Contemplation" , "Goddess; rock group" , "The source of an artist's inspiration" , "Source of artistic inspiration" .)