British monarch keeps painting in cashless deal (6)
I believe the answer is:
barter
'cashless deal' is the definition.
(I've seen this in another clue)
'british monarch keeps painting' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'keeps' indicates putting letters inside (inserted letters are kept inside).
'painting' becomes 'art' (painting is an art).
'b'+'er'='ber'
'ber' going around 'art' is 'BARTER'.
'in' acts as a link.
(Other definitions for barter that I've seen before include "Trade by exchanging commodities" , "Trade by direct exchange of goods" , "Trade not involving money" , "Trade or swap" , "Exchange, bargain" .)