Central idea in anthem expressed (5)
I believe the answer is:
theme
'central idea' is the definition.
(I've seen this in another clue)
'in anthem expressed' is the wordplay.
'in' indicates the answer is hidden within the clue.
'THEME' can be found hidden inside 'anthem expressed'.
(Other definitions for theme that I've seen before include "idea of the work" , "Subject set for discussion" , "Signature tune" , "Subject; recurring idea" , "subject,motif" .)