Mature female let down (6)
I believe the answer is:
flower
'mature' is the definition.
(thesaurus)
'female let down' is the wordplay.
'female' becomes 'f' (common abbreviation).
'let down' becomes 'lower' (synonyms).
'f'+'lower'='FLOWER'
(Other definitions for flower that I've seen before include "One found in bed perhaps" , "prime of life" , "rose, perhaps" , "Garden plant" , "River" .)