Depression is principally the subject for medical specialists (8)
I believe the answer is:
dentists
'medical specialists' is the definition.
The definition and answer can be both people as well as being plural nouns.
Perhaps you can see a link between them that I can't see?
'depression is principally the subject' is the wordplay.
'depression' becomes 'dent' (synonyms).
'principally' indicates taking the first letters.
The first letters of 'the subject' is 'ts'.
'dent'+'is'+'ts'='DENTISTS'
'for' is the link.
Can you help me to learn more?
(Other definitions for dentists that I've seen before include "Orthodontists, periodontists etc." , "Tooth doctors" , "They care for teeth" , "Professional carers for teeth" .)