Charge girl for herbaceous plant (8)
I believe the answer is:
costmary
'herbaceous plant' is the definition.
(costmary is a kind of herbaceous plant)
'charge girl' is the wordplay.
'charge' becomes 'cost' (I've seen this in other clues).
'girl' becomes 'mary' (**).
'cost'+'mary'='COSTMARY'
'for' acts as a link.
(Other definitions for costmary that I've seen before include "Aromatic plant" , "Herb" .)