Jacket around back of organ (7)
I believe the answer is:
cardiac
'back of organ' is the definition.
Although both the answer and definition are adjectives, I don't understand how they can define each other.
'jacket around' is the wordplay.
'jacket' becomes 'cardia' (I can't justify this - if you can you should believe this answer much more).
'around' becomes 'c' (abbreviation for 'circa').
'cardia'+'c'='CARDIAC'
Can you help me to learn more?
(Other definitions for cardiac that I've seen before include "Appertaining to the heart" , "of hearts" , "Relating to 8 down, part one" , "Of the heat, perhaps illness" , "Sort of surgery" .)