Wasted this amount over strip of land (7)
I believe the answer is:
isthmus
'strip of land' is the definition.
(I've seen this before)
'wasted this amount over' is the wordplay.
'wasted' indicates anagramming the letters.
'amount' becomes 'sum' (synonyms).
'over' says the letters should be written in reverse.
'sum' written backwards gives 'mus'.
'this' is an anagram of 'isth'.
'isth'+'mus'='ISTHMUS'
(Other definitions for isthmus that I've seen before include "Constriction" , "Neck of land connecting larger areas" , "Narrow strip of land between two larger ones" , "Narrow strip linking two larger areas of land" , "Narrow neck of land joining two larger portions" .)