Time doctor treated a mere idler (10)
I believe the answer is:
daydreamer
'idler' is the definition.
(daydreamer is a kind of idler)
'time doctor treated a mere' is the wordplay.
'time' becomes 'day' (day is a kind of time).
'doctor' becomes 'dr' (abbreviation).
'treated' indicates anagramming the letters.
'a'+'mere'='amere'
'amere' with letters rearranged gives 'eamer'.
'day'+'dr'+'eamer'='DAYDREAMER'
(Other definitions for daydreamer that I've seen before include "One in a reverie" , "Wishful thinker" , "one in a brown study" , "One distracted from the present" , "one has unrealistic expectations" .)