One in a mount would be a valuable picture (5)
I believe the answer is:
monet
'would be a valuable picture' is the definition.
I can't tell whether this definition defines the answer.
'one in a mount' is the wordplay.
'in' indicates putting letters inside.
'a mount' becomes 'mt'.
'one' going inside 'mt' is 'MONET'.
Can you help me to learn more?
(Other definitions for monet that I've seen before include "French landscape painter" , "Met no impressionist painter" , "he painted Rouen Cathedral" , "French artist shown in ..." , "Claude -, French artist" .)