Trees covering space by northern English hills (8)
I believe the answer is:
pennines
'english hills' is the definition.
'trees covering space by northern' is the wordplay.
'trees' becomes 'pines' (pine is a kind of tree**).
'covering' is an insertion indicator.
'space' becomes 'en'.
'by' means one lot of letters go next to another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'en'+'n'='enn'
'pines' enclosing 'enn' is 'PENNINES'.
(Other definitions for pennines that I've seen before include "English mountain range" , "Range of hills in England" , "Backbone of England" , "chain of hills" .)