Alternatively gone off with a herb (7)
I believe the answer is:
oregano
'herb' is the definition.
(oregano is a kind of herb)
'alternatively gone off with a' is the wordplay.
'alternatively' becomes 'or'.
'off with' is an anagram indicator.
'gone' with its letters mixed up with 'a' makes 'egano'.
'or'+'egano'='OREGANO'
(Other definitions for oregano that I've seen before include "Aromatic culinary herb" , "that's aromatic" , "Dried marjoram" , "Herb used in cookery" , "Kitchen herb" .)