Cook particular course, speciality (6)
I believe the answer is:
domain
'speciality' is the definition.
(thesaurus)
'cook particular course' is the wordplay.
'cook' becomes 'do' (do can mean to cook).
'particular course' becomes 'main' (main course is a kind of course. I am not sure about the 'particular' bit.).
'do'+'main'='DOMAIN'
Can you help me to learn more?
(Other definitions for domain that I've seen before include "Field of thought or activity" , "Range of influence or field of activity" , "estate" , "A district sphere of influence" , "Range of influence or field of thought" .)