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 can't see how one could define the other.
'mother and son' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'and' means one lot of letters go next to another.
'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" .)