Study on time with first tooth specialist (7)
I believe the answer is:
dentist
'tooth specialist' is the definition.
(I've seen this before)
'study on time with first' is the wordplay.
'study' becomes 'den' (term for a study or office).
'on' is a charade indicator (letters next to each other).
'time' becomes 't' (abbreviation).
'with' means one lot of letters go next to another.
'first' becomes 'ist' (ist resembles 1st).
'den'+'t'+'ist'='DENTIST'
(Other definitions for dentist that I've seen before include "Oral surgeon" , "Someone who drills for a living?" , "Tooth expert" , "Teeth treater" , "Repairer of teeth" .)