Impertinent, say, about saints (5)
I believe the answer is:
sassy
'impertinent' is the definition.
(thesaurus)
'say about saints' is the wordplay.
'about' indicates putting letters inside.
'saints' becomes 'ss'.
'say' placed around 'ss' is 'SASSY'.
(Other definitions for sassy that I've seen before include "Pert" , "Bold, spirited, cheeky (Amer.)" , "Impertinent (US colloq.)" , "Cheeky American" , "(In US) saucy" .)