Prevent notes upsetting liar (6)
I believe the answer is:
derail
'prevent' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how one could define the other.
'notes upsetting liar' is the wordplay.
'notes' becomes 'de' (two musical notes).
'upsetting' indicates an anagram.
'liar' is an anagram of 'rail'.
'de'+'rail'='DERAIL'
(Other definitions for derail that I've seen before include "get off the line" , "Of train, force off tracks" , "stop things going smoothly" , "create an obstructive diversion" , "Force off track, like a train" .)