Mother and son, possibly free (5)
I believe the answer is:
mason
'free' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'mother and son' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'and' is a charade indicator (letters next to each other).
'ma'+'son'='MASON'
'possibly' acts as a link.
Can you help me to learn more?
(Other definitions for mason that I've seen before include "One in lodge" , "One going to work in building" , "Jewish comedian" , "one in construction" , "one wearing an apron" .)