Lazy siesta beneath the cover of Monsieur Henri? (7)
I believe the answer is:
matisse
'henri?' is the definition.
(painter Henri Matisse, 1869-1954)
'lazy siesta beneath the cover of monsieur' is the wordplay.
'lazy' is an anagram indicator (a lazy or relaxed spelling).
'beneath the cover of' means one lot of letters go next to another (in a down clue, some letters go below others).
'monsieur' becomes 'm' (French abbreviation - like Mr for Mister).
'siesta' with letters rearranged gives 'atisse'.
'atisse' after 'm' is 'MATISSE'.
(Other definitions for matisse that I've seen before include "Mae sits badly for the French painter" , "Henri ---, Fauvist painter" , "French painter, Henri, d.1954" , "Henri -, Fr. artist" , "Is mates turned on the French painter" .)