Saints clad in undies kiss cheek (5,4)
I believe the answer is:
brass neck
'cheek' is the definition.
(both can mean insolence or rudeness)
'saints clad in undies kiss' is the wordplay.
'saints' becomes 's' (this might be a standard abbreviation of which I'm unaware).
'clad in' is an insertion indicator.
'undies' becomes 'bras' (I've seen this in another clue).
'kiss' becomes 'neck' (neck can informally mean to kiss).
's' going inside 'bras' is 'brass'.
'brass'+'neck'='BRASS NECK'
Can you help me to learn more?
(Other definitions for brass neck that I've seen before include "Effrontery" , "Offering cheek" .)