Flower completes Monet's first artwork (5)
I believe the answer is:
mural
'artwork' is the definition.
(I've seen this before)
'flower completes monet's first' is the wordplay.
'flower' becomes 'ural' (I've seen this before).
'completes' says to put letters next to each other (finalises the answer by going on the end).
'monet's first' becomes 'm' (ust letter of 'monet').
'ural' put after 'm' is 'MURAL'.
(Other definitions for mural that I've seen before include "Drawing on a wall" , "Large wall painting" , "Wall decoration" , "Wall art" , "Artwork" .)