Related fact ultimately obsolete (4)
I believe the answer is:
told
'related' is the definition.
(I've seen this before)
'fact ultimately obsolete' is the wordplay.
'ultimately' says to take the final letters.
'obsolete' becomes 'old' (I've seen this before).
The final letter of 'fact' is 't'.
't'+'old'='TOLD'
(Other definitions for told that I've seen before include "Authorised - broadcast" , "instructed" , "Made an impact" , "Related (a story)" , "Produced effect; related" .)