Cook may get sweet on him! (5)
I believe the answer is:
basil
'cook may' is the definition.
The answer and definition can be both plants as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'get sweet on him' is the wordplay.
'get sweet' becomes 'as' (I can't explain this - if you can you should believe this answer much more).
'on' is an insertion indicator.
'him' becomes 'Bil'.
'as' placed into 'bil' is 'BASIL'.
Can you help me to learn more?
(Other definitions for basil that I've seen before include "Herb of mint family" , "serially chaotic character" , "A herb; Cardinal --- Hume" , "Culinary herb" , "Sweet-smelling herb" .)