State of an atom, spinning around neutron initially (7)
I believe the answer is:
montana
'state' is the definition.
(state of the US)
'an atom spinning around neutron initially' is the wordplay.
'spinning' shows that the letters should be reversed in order.
'around' means one lot of letters goes inside another.
'initially' suggests taking the first letters.
The first letter of 'neutron' is 'n'.
'an'+'atom'='anatom'
'anatom' in reverse letter order is 'motana'.
'motana' enclosing 'n' is 'MONTANA'.
'of' acts as a link.
(Other definitions for montana that I've seen before include "North-western US state" , "State N. of Wyoming" , "The "Big Sky State"" , "American state, capital Helena" , "One US state ..." .)