Study with a headmaster (4)
I believe the answer is:
dean
'headmaster' is the definition.
(thesaurus)
'study with a' is the wordplay.
'study' becomes 'den' (term for a study or office).
'with' means one lot of letters goes inside another.
'den' placed around 'a' is 'DEAN'.
(Other definitions for dean that I've seen before include "Small valley - cathedral dignitary" , "College officer" , "Senior academic administrator" , "Jonathan Swift's occupation" , "Kind of clergyman or university official" .)