Smothering the unduly deceitful (9)
I believe the answer is:
overlying
'smothering' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
'unduly deceitful' is the wordplay.
'unduly' becomes 'over' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'deceitful' becomes 'lying' (I've seen this in another clue).
'over'+'lying'='OVERLYING'
'the' is the link.
Can you help me to learn more?
(Another definition for overlying that I've seen is " Placed on top of".)