Land bridge exists as shown, covering miles (7)
I believe the answer is:
isthmus
'land bridge' is the definition.
(strip of land linking two larger areas)
'exists as shown covering miles' is the wordplay.
'exists' becomes 'is' (if something 'is', it exists).
'as shown' becomes 'thus' ('thus' can mean 'like this').
'covering' means one lot of letters goes inside another.
'miles' becomes 'm' (common abbreviation).
'thus' enclosing 'm' is 'thmus'.
'is'+'thmus'='ISTHMUS'
(Other definitions for isthmus that I've seen before include "Narrow strip of land connecting two larger areas" , "Neck of land between two seas" , "Narrow land link" , "Narrow strip of land with water on two sides" , "Tongue" .)