Flag to which there is a long story (4)
I believe the answer is:
saga
'there is a long story' is the definition.
(I've seen this before)
'flag to' is the wordplay.
'flag' becomes 'sag' (synonyms).
'to' becomes 'a' (I can't explain this - if you can you should give a lot more credence to this answer).
'sag'+'a'='SAGA'
'which' is the link.
Can you help me to learn more?
(Other definitions for saga that I've seen before include "Long involved tale" , "Norse epic" , "long series of events" , "Long detailed story" , "A long adventurous story - it's a gas" .)