An obvious untruth on record (4)
I believe the answer is:
disc
'record' is the definition.
('disc' can be a synonym of 'record')
'an obvious untruth on' is the wordplay.
'an' becomes 'i' (Roman numeral for one).
'obvious untruth' becomes 'dsc' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'i' placed within 'dsc' is 'DISC'.
Can you help me to learn more?
(Other definitions for disc that I've seen before include "Flat round plate" , "Spine cartilage layer" , "One of 33, 45 or 78?" , "CD perhaps" , "Forty-five" .)