Yours truly and child - a small charge (5)
I believe the answer is:
meson
'a small charge' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'yours truly and child' is the wordplay.
'yours truly' becomes 'me' (the person that set this crossword).
'and' means one lot of letters go next to another.
'child' becomes 'son' (male child).
'me'+'son'='MESON'
Can you help me to learn more?
(Other definitions for meson that I've seen before include "tiny particle" , "Quark/antiquark particle" , "small particle" , "Subatomic particle" , "Elementary thing" .)