Borderline impudence I heard at intervals (8)
I believe the answer is:
frontier
'borderline' is the definition.
(thesaurus)
'impudence i heard at intervals' is the wordplay.
'impudence' becomes 'front' (I've seen this before).
'at intervals' indicates alternate letters (letters are taken at regular intervals).
'i'+'heard'='iheard'
The alternating letters of 'iheard' are 'ier'.
'front'+'ier'='FRONTIER'
(Other definitions for frontier that I've seen before include "Border (not yet crossed?)" , "Border separating two countries" , "Limit" , "International boundary" , "Outer edge of a settled territory" .)