Was absorbed in thought (5)
I believe the answer is:
mused
'was absorbed in thought' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mused that I've seen before include "Was absorbed in thought" , "Thought (to oneself)" , "Reflected in silence" , "'Meditated, mulled (5)'" , "Was deep in thought" .)