Doctor then, I see, concerned with race (6)
I believe the answer is:
ethnic
'race' is the definition.
(I've seen this before)
'doctor then i see' is the wordplay.
'doctor' indicates anagramming the letters.
'see' becomes 'C' (the word for the letter, according to Chambers).
'then' is an anagram of 'ethn'.
'ethn'+'i'+'c'='ETHNIC'
'concerned with' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ethnic that I've seen before include "Relating to cultures" , "To do with racial group or culture" , "Relating to a cultural tradition" , "To do with race or nationality" , "Etch in (anag.)" .)