Could you call him Herb? (5)
I believe the answer is:
basil
'herb?' is the definition.
(basil is a kind of herb)
'could you call him' is the wordplay.
I cannot quite see how this works, but
'him' could be 'si' and 'si' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for basil that I've seen before include "Boy - herb" , "Culinary herb" , "Plant; Brush; Fawlty" , "Aromatic herb with purplish-green leaves" , "Herb - Mr Fawlty" .)