Assume he's a chap (3)
I believe the answer is:
don
This clue is a triple definition!
'assume' is the first definition.
(I know that assume can be written as don)
'he's' is the second definition.
'a chap' is the third definition.
(Other definitions for don that I've seen before include "Spanish male title" , "Spanish nobleman" , "University lecturer" , "Spanish gent or Mafia chief" , "- - Giovanni, 26" .)