Maybe Rosemary's Baby's opening after fuss (7)
I believe the answer is:
potherb
'maybe rosemary's' is the definition.
(rosemary is an example)
'baby's opening after fuss' is the wordplay.
'opening' says to take the initial letters.
'after' means one lot of letters go next to another.
'fuss' becomes 'pother' (pothering is a kind of fussing).
The first letter of 'babys' is 'b'.
'b' after 'pother' is 'POTHERB'.
(Other definitions for potherb that I've seen before include "Plant grown for culinary use" , "Useful kitchen plant" , "this for season" , "Plant, parts of which are used in cooking" , "Plant used for seasoning in cooking" .)