Artists with a mental image, not half an uncommon thing (4,4)
I believe the answer is:
rara avis
'uncommon thing' is the definition.
(I've seen this in another clue)
'artists with a mental image not half an' is the wordplay.
I cannot really see how this works, but
'artists' could be 'ra' (Royal Academy of the Arts) and 'ra' is found within the answer.
'a' is within the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rara avis that I've seen before include "An uncommon thing" , "Unique, or seldom encountered, person, to the old Romans" , "An unusual person, or bird" , "Uncommon person/thing" , "curiosity" .)