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 see how they can define each other.
'mother and son' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'and' says to put letters next to each other.
'ma'+'son'='MASON'
'possibly' is the 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" .)