Core things were initially in a state of rest (7)
I believe the answer is:
inertia
'core things were initially' is the definition.
I can't judge whether this definition defines the answer.
'in a state of rest' is the wordplay.
'in' means one lot of letters goes inside another.
'a state' becomes 'IA'.
'of rest' becomes 'nerti' (I can't justify this - if you can you should give a lot more credence to this answer).
'ia' enclosing 'nerti' is 'INERTIA'.
Can you help me to learn more?
(Other definitions for inertia that I've seen before include "Resistance to motion" , "Disinclination to respond" , "Tendency not to act" , "Admonish" , "I retain, being without energy" .)