One turns, endlessly spinning particle (7)
I believe the answer is:
neutron
'particle' is the definition.
(neutron is a kind of particle)
'one turns endlessly spinning' is the wordplay.
'endlessly' means to remove the last letter.
'spinning' is an anagram indicator.
'one'+'turns'='oneturns'
'oneturns' with its last letter removed is 'oneturn'.
'oneturn' anagrammed gives 'NEUTRON'.
(Other definitions for neutron that I've seen before include "Chargeless particle" , "Turn one (anag) - kind of bomb" , "Charged particle" , "one's not charged" , "Uncharged particle in a nucleus" .)