Impressionist one entering mountain (5)
I believe the answer is:
monet
'impressionist' is the definition.
(I've seen this before)
'one entering mountain' is the wordplay.
'entering' indicates putting letters inside.
'mountain' becomes 'mt'.
'one' put into 'mt' is 'MONET'.
(Other definitions for monet that I've seen before include "French Impressionist with famous garden" , "Claude, impressionist painter, d. 1926" , "French artist shown in ..." , "'Water Lilies' 13ac" , "Met no impressionist painter" .)