Surround eastern and northern part of theatre (8)
I believe the answer is:
encircle
'surround' is the definition.
('encircle' can be a synonym of 'surround')
'eastern and northern part of theatre' is the wordplay.
'eastern' becomes 'e'.
'and' means one lot of letters go next to another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'part of theatre' becomes 'circle' ('circle' is part of 'theatre').
'e'+'n'+'circle'='ENCIRCLE'
(Other definitions for encircle that I've seen before include "Contain" , "Ring round" , "Surround" .)