Boast of rupees in sack (4)
I believe the answer is:
brag
'boast' is the definition.
(I know that brag is a type of boast)
'rupees in sack' is the wordplay.
'rupees' becomes 'r' (abbreviation for rupees).
'in' indicates putting letters inside.
'sack' becomes 'bag' (I've seen this in another clue).
'r' put within 'bag' is 'BRAG'.
'of' acts as a link.
(Other definitions for brag that I've seen before include "Boast about garb" , "Card game - boast" , "Boast, exaggerate" , "Idle talk" , "Boast, crow" .)