A woman; one about 51 (5)
I believe the answer is:
alice
'a woman' is the definition.
'one about 51' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'about' indicates putting letters inside.
'5a' becomes 'LI' (Roman numerals).
'ace' placed around 'li' is 'ALICE'.
(Other definitions for alice that I've seen before include "Girl famed for Wonderland adventures" , "Carroll heroine" , "Wonderland heroine" , "Miss Liddell" , "Girl famous for entering Wonderland" .)