Doctor having good thought (5)
I believe the answer is:
mused
'thought' is the definition.
(musing is a kind of thinking)
'doctor having good' is the wordplay.
'doctor' becomes 'md' (synonyms).
'having' is an insertion indicator.
'good' becomes 'use' (in phrases like 'it's no good', 'any good?' etc).
'md' enclosing 'use' is 'MUSED'.
(Other definitions for mused that I've seen before include "Thought (to oneself)" , "Was deep in thought" , "Reflected or pondered" , "Was absorbed in thought" , "Closer" .)