Barking MP's irate over opening of nuclear plant (9)
I believe the answer is:
spearmint
'plant' is the definition.
(I know that spearmint is a type of Herb)
'barking mp's irate over opening of nuclear' is the wordplay.
'barking' is an anagram indicator ('barking' can informally mean mad).
'over' is an insertion indicator.
'opening of' suggests taking the first letters.
The initial letter of 'nuclear' is 'n'.
'mps'+'irate'='mpsirate'
'mpsirate' with letters rearranged gives 'spearmit'.
'spearmit' placed around 'n' is 'SPEARMINT'.
(Other definitions for spearmint that I've seen before include "fresh flavour" , "Herb used for flavouring" , "Garden flavouring" , "Strongly flavoured herb" , "Aromatic herb" .)