One turn may be free of charge (7)
I believe the answer is:
neutron
'free of charge' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'one turn may be' is the wordplay.
'may be' indicates an anagram.
'one'+'turn'='oneturn'
'oneturn' anagrammed gives 'NEUTRON'.
Can you help me to learn more?
(Other definitions for neutron that I've seen before include "Uncharged subatomic particle" , "Uncharged particle in a nucleus" , "Turn one (anag) - kind of bomb" , "On tuner (anag.)" , "Elementary particle with zero charge" .)