It's an arm, not the head, that adapts slowly (7)
I believe the answer is:
article
This could be a double definition.
'it's' is the first definition.
(I've seen this before)
'an arm not the head that adapts slowly' is the second definition.
'article' can be an answer for 'an'. I am not certain of the rest of the definition.
Can you help me to learn more?
(Other definitions for article that I've seen before include "Feature - item" , "See 10" , "Newspaper item" , "What journalist writes" , "Object - piece of writing" .)