They're of fundamental concern (6)
I believe the answer is:
basics
'they're' is the definition.
The definition suggests an adverb but the answer is not.
'fundamental concern' is the wordplay.
'fundamental' becomes 'basic' (synonyms).
'concern' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'basic'+'s'='BASICS'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for basics that I've seen before include "Rudiments" , "Fundamentals, starting-points" , "Essential principles" .)