It's a long story (4)
'long story' is the definition.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for yarn that I've seen before include "Story, and thread or wool, say" , "Long involved story" , "Rope" , "Rambling story" , "Spun thread; long story" .)
'long story' is the definition.
(a saga is a long and complicated story)
'it's a' is the wordplay.
'it's' becomes 'sag' (I can't explain this - if you can you should believe this answer much more).
'sag'+'a'='SAGA'
Can you help me to learn more?
(Other definitions for saga that I've seen before include "Epic tale" , "Chronicle; adventure" , "Norse epic" , "Lengthy tale" , "Epic - legend" .)