Endlessly observe Magritte's calm (6)
I believe the answer is:
serene
'calm' is the definition.
(synonyms)
'endlessly observe magritte's' is the wordplay.
'endlessly' means to remove the last letter.
'observe' becomes 'see' (I've seen this in other clues).
'magritte' becomes 'rene' (artist Rene Magritte).
'see' with its final letter taken away is 'se'.
'se'+'rene'='SERENE'
(Other definitions for serene that I've seen before include "Peaceful and contented" , "Peaceful and calm in manner" , "Calm and relaxed" , "Calm, peaceful, tranquil" , "Calm and composed" .)