Lie, untruth (3)
I believe the answer is:
fib
I believe this clue is a double definition.
'lie' is the first definition.
(I know that fib is a type of lie)
'untruth' is the second definition.
(fib is a kind of untruth)
(Other definitions for fib that I've seen before include "A harmless white lie" , "Story" , "Little white lie" , "Trivial lie" , "One must be 20 [UNTRUE]" .)