Doctor concerned with substance not entirely uncommon (9)
I believe the answer is:
registrar
'doctor' is the definition.
(I've seen this before)
'concerned with substance not entirely uncommon' is the wordplay.
'concerned with' becomes 're'.
'substance' becomes 'gist' (gist is a kind of substance**).
'not entirely' means to remove the last letter.
'uncommon' becomes 'rare' ('rare' can be similar in meaning to 'uncommon').
'rare' with its last letter taken off is 'rar'.
're'+'gist'+'rar'='REGISTRAR'
(Other definitions for registrar that I've seen before include "University administrator -- hospital doctor" , "Mid-ranking doctor" , "Administrator responsible for records" , "clerk" , "Official record keeper" .)