Age of a noted regiment (3)
I believe the answer is:
era
'age' is the definition.
(both can mean a long time)
'noted regiment' is the wordplay.
I cannot quite understand how this works, but
'regiment' could be 'ra' (Royal Artillery) and 'ra' is found within the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'of a' is the link.
Can you help me to learn more?
(Other definitions for era that I've seen before include "Age marked by a distinctive character" , "A particular period of time" , "8" , "Are involved in time span" , "Aeon" .)