Brown's sanctimonious in the main (5)
I believe the answer is:
sepia
'brown's' is the definition.
(sepia is a kind of brown)
'sanctimonious in the main' is the wordplay.
'sanctimonious' becomes 'pi'.
'in the' is an insertion indicator.
'main' becomes 'sea' ('sea' can be a synonym of 'main').
'pi' going within 'sea' is 'SEPIA'.
(Other definitions for sepia that I've seen before include "Old drawing" , "Colour from cuttlefish ink" , "Pigment obtained from cuttlefish ink" , "Shade of brown, as in old photographs" , "Brown colour of old photo" .)