It's an old story (4)

I believe the answer is:

saga

I'm a little stuck... Click here to teach me more about this clue!

'old story' is the definition.
(I've seen this before)

'it's an' is the wordplay.
'it's' becomes 'sag' (I can't explain this - if you can you should give a lot more credence to this answer).
'an' becomes 'a'.
'sag'+'a'='SAGA'

Can you help me to learn more?

(Other definitions for saga that I've seen before include "Long and involved story" , "Heroic narrative" , "Lengthy tale" , "Epic story" , "Norse epic" .)

I've seen this clue in the Evening Standard.

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