Hiding the fact that untruthfulness is despicable (5,3)
I believe the answer is:
lying low
'hiding the fact' is the definition.
'lying low' can be an answer for 'hiding' (lying low is a kind of hiding). I am not certain of the 'the fact' bit.
'untruthfulness is despicable' is the wordplay.
'untruthfulness' becomes 'lying' (I've seen this in another clue).
'is despicable' becomes 'low' (synonyms).
'lying'+'low'='LYING LOW'
'that' is the link.
Can you help me to learn more?
(Other definitions for lying low that I've seen before include "Avoiding attention" , "Concealed" .)