Woman’s name concealed by mature son (5)
I believe the answer is:
agnes
'woman's' is the definition.
'name concealed by mature son' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'concealed by' indicates putting letters inside.
'mature' becomes 'age' (synonyms).
'son' becomes 's'.
'n' inserted within 'age' is 'agne'.
'agne'+'s'='AGNES'
(Other definitions for agnes that I've seen before include "Patron saint of virgins" , "St --' Eve, poem by Lord Tennyson" , "Miss Grey" , "The Eve of St. - (Keats)" , "David's second wife" .)