Epic decline, first in ages (4)
I believe the answer is:
saga
'epic' is the definition.
(thesaurus)
'decline first in ages' is the wordplay.
'decline' becomes 'sag' (I've seen this before**).
'first in' suggests taking the first letters.
The first letter of 'ages' is 'a'.
'sag'+'a'='SAGA'
(Other definitions for saga that I've seen before include "Flag associated with a" , "Soap opera" , "Norse epic" , "Icelandic or Norse story" , "Norse narrative" .)