Trade Union releases aid - it's free of charge (7)
I believe the answer is:
neutron
'charge' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'trade union releases aid it's free' is the wordplay.
'releases' suggests deleting specific letters.
'it's free' is an anagram indicator.
'trade'+'union'='tradeunion'
'tradeunion' with 'aid' taken out is 'treunon'.
'treunon' with letters rearranged gives 'NEUTRON'.
'of' is the link.
Can you help me to learn more?
(Other definitions for neutron that I've seen before include "Particle that decays when released" , "tiny amount" , "Uncharged atomic particle" , "Charged particle" , "Uncharged subatomic particle" .)