British historian? Lay off this Chinese region! (5)
I believe the answer is:
macau
'this chinese region' is the definition.
'british historian? lay off' is the wordplay.
'british historian?' becomes 'macaulay' (Thomas Babington Macaulay).
'off' indicates named letters should be taken away.
'macaulay' with 'lay' removed is 'MACAU'.
(Other definitions for macau that I've seen before include "Small peninsula in China, a Portuguese territory until 1999" , "somewhere in China" , "SW of 24 [SHANGHAI]" , "region of China" , "Part of China" .)