Thought doctor to be without practice (5)
I believe the answer is:
mused
'thought' is the definition.
(musing is a kind of thinking)
'doctor to be without practice' is the wordplay.
'doctor' becomes 'md' (synonyms).
'to be without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'practice' becomes 'use' (use is a kind of practice).
'md' enclosing 'use' is 'MUSED'.
(Other definitions for mused that I've seen before include "'Meditated, mulled (5)'" , "Contemplated" , "Reflected in silence" , "Reflected or pondered" , "Was deep in thought" .)