Herb who is married to Sybil (5)
I believe the answer is:
basil
'herb' is the definition.
(basil is a herb)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for basil that I've seen before include "Pesto ingredient" , "Herb of mint family" , "Man" , "Culinary plant" , "Bails out the herb" .)