Nick is beginning to train as a tooth specialist (7)
I believe the answer is:
dentist
'a tooth specialist' is the definition.
(I've seen this before)
'nick is beginning to train' is the wordplay.
'nick' becomes 'dent' (synonyms).
'beginning to' indicates taking the first letters.
The first letter of 'train' is 't'.
'dent'+'is'+'t'='DENTIST'
'as' is the link.
(Other definitions for dentist that I've seen before include "Repairer of teeth" , "Tooth specialist" , "Open wide for him" , "One treating teeth" , "Person qualified to treat teeth - tends it (anag)" .)