Rosemary? That lady's bold (4)
I believe the answer is:
herb
'rosemary?' is the definition.
(rosemary is a kind of herb)
'lady's bold' is the wordplay.
'lady' becomes 'her' (her refers to a woman).
'bold' becomes 'b' (abbreviation).
'her'+'b'='HERB'
'that' acts as a link.
(Other definitions for herb that I've seen before include "Plant used for flavour" , "Useful or edible plant" , "man's name" , "Rosemary perhaps" , "Wayside plant with small red flowers" .)