Old impressionist wanting last piece mounted (4)
I believe the answer is:
aged
'old' is the definition.
(similar in meaning)
'impressionist wanting last piece mounted' is the wordplay.
'impressionist' becomes 'age' (I can't explain this - if you can you should give a lot more credence to this answer).
'wanting' means one lot of letters go next to another.
'last piece' suggests the final letters (I've seen 'last piece of' mean this).
The final letter of 'mounted' is 'd'.
'age'+'d'='AGED'
Can you help me to learn more?
(Other definitions for aged that I've seen before include "Became older" , "Senior" , "Antiquated" , "not like a callow youth" , "Ripened, matured" .)