Made up awful tune introducing rugby union (6)
I believe the answer is:
untrue
'made up' is the definition.
(I've seen this in another clue)
'awful tune introducing rugby union' is the wordplay.
'awful' indicates anagramming the letters.
'introducing' is an insertion indicator (some letters introduced or inserted into others).
'rugby union' becomes 'RU' (abbreviation).
'tune' anagrammed gives 'unte'.
'unte' going around 'ru' is 'UNTRUE'.
(Other definitions for untrue that I've seen before include "Counter-factual" , "Legendary" , "Not correct, like a lie" , "Not factual or honest" , "Not faithful" .)