One shown up in mendacity?! (3)
I believe the answer is:
cad
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'shown up in mendacity?' is the wordplay.
'shown up' says the letters should be written backwards.
'in' says the answer is hidden in the clue.
'dac' is hidden within 'mendacity'.
'dac' reversed gives 'CAD'.
Can you help me to learn more?
(Other definitions for cad that I've seen before include "Ungentlemanly person" , "Rogue" , "Dishonourable man of decadent sort" , "he's no gent" , "Rotter, bounder" .)