Painter’s instant source of information (5)
I believe the answer is:
monet
'painter's' is the definition.
(artist Claude Monet)
'instant source of information' is the wordplay.
'instant' becomes 'mo'.
'source of information' becomes 'net' (the internet is a source of information).
'mo'+'net'='MONET'
(Other definitions for monet that I've seen before include "he painted Rouen Cathedral" , "Claude -, French artist" , "Claude, impressionist painter, d. 1926" , "French Impressionist with famous garden" , "Claude ---, painter (waterlilies)" .)