Untruth with Lee one concocted (5,3)
I believe the answer is:
white lie
'untruth' is the definition.
(I have seen 'Harmless untruth ' mean 'white lie' so perhaps 'untruth' could also mean 'white lie')
'with lee one concocted' is the wordplay.
'one' becomes 'I' (Roman numeral).
'concocted' indicates anagramming the letters.
'with'+'lee'+'i'='withleei'
'withleei' with letters rearranged gives 'WHITE LIE'.
(Other definitions for white lie that I've seen before include "Supposedly helpful untruth" , "Minor falsehood" , "Harmless untruth" , "Justifiable deception" , "Something untrue, but forgivable" .)