Woman with a son about 51 (6)
I believe the answer is:
alison
'woman' is the definition.
'a son about 51' is the wordplay.
'about' means one lot of letters go next to another.
'51' becomes 'li' (roman numeral).
'son' put after 'li' is 'lison'.
'a'+'lison'='ALISON'
'with' acts as a link.
(Other definitions for alison that I've seen before include "Girl's name" , "Girl" , "lady" , "woman" , "female" .)