One's so covering miles in such a strip of land (7)
I believe the answer is:
isthmus
'such a strip of land' is the definition.
(I've seen this before)
'one's so covering miles' is the wordplay.
'one's' becomes 'is' (roman numeral).
'so' becomes 'thus' ('thus' can be a synonym of 'so'**).
'covering' is an insertion indicator.
'miles' becomes 'm' (common abbreviation).
'thus' enclosing 'm' is 'thmus'.
'is'+'thmus'='ISTHMUS'
'in' is the link.
(Other definitions for isthmus that I've seen before include "Narrow land bridge" , "Narrow linking piece of land" , "Constriction" , "Narrow strip of land connecting two large masses" , "Strip of land flanked by sea" .)