Alternatively gone mad about a herb (7)
I believe the answer is:
oregano
'herb' is the definition.
(I know that oregano is a type of herb)
'alternatively gone mad about a' is the wordplay.
'alternatively' becomes 'or' ().
'mad' indicates anagramming the letters.
'about' indicates putting letters inside.
'gone' with letters rearranged gives 'egno'.
'egno' enclosing 'a' is 'egano'.
'or'+'egano'='OREGANO'
(Other definitions for oregano that I've seen before include "Pungent seasoning leaves" , "Gear Ono up for the herb" , "Sweet-smelling herb" , "food flavouring" , "leaves for season" .)