Chain fences round square (8)
I believe the answer is:
pennines
'chain' is the definition.
(Pennine Chain, hills in Northern England)
'fences round square' is the wordplay.
'fences' becomes 'pens' (pen can mean to enclose or fence in).
'round' means one lot of letters goes inside another.
'square' becomes 'nine' (I've seen this before).
'pens' placed around 'nine' is 'PENNINES'.
(Other definitions for pennines that I've seen before include "chain of hills" , "Backbone of England" , "Range of hills in England" , "English mountain range" .)