Herb succeeded to breach security (5)
I believe the answer is:
basil
'herb' is the definition.
(basil is a herb)
'succeeded to breach security' is the wordplay.
I cannot quite understand how this works, but
'succeeded' could be 's' (genealogical abbreviation) and 's' is found within the answer.
'security' could be 'bail' (I've seen this in another clue) and 'bail' is found within the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for basil that I've seen before include "Doctor of the fourth century" , "A herb; --- Brush, Fawlty" , "Herb (from Fawlty Towers?)" , "Fragrant herb" , "Mediterranean herb" .)