Oddly flawless bottom line is not true (5)
I believe the answer is:
false
'is not true' is the definition.
('false' is the opposite to 'true')
'oddly flawless bottom line' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'bottom' suggests the final letters (I've seen 'bottom of' mean this).
The last letter of 'line' is 'e'.
The alternating letters of 'flawless' are 'fals'.
'fals'+'e'='FALSE'
(Other definitions for false that I've seen before include "Not genuine or real" , "Fake; untrue" , "Forged" , "Untrue - treacherous - counterfeit" , "Spurious, untrue" .)