Help Max Ernst to hold up illustration (7)
I believe the answer is:
example
'illustration' is the definition.
('example' can be a synonym of 'illustration')
'help max ernst to hold up' is the wordplay.
'to hold' indicates the answer is hidden within the clue (I've seen 'held' mean this).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'elpmaxe' can be found hidden inside 'help max ernst'.
'elpmaxe' reversed gives 'EXAMPLE'.
(Other definitions for example that I've seen before include "Specimen, illustration" , "Illustration or model" , "Illustrative exercise" , "Characteristic item" , "Fact that illustrates a general rule" .)