This unusual Greek character's strip of land (7)
I believe the answer is:
isthmus
'strip of land' is the definition.
(I've seen this before)
'this unusual greek character's' is the wordplay.
'unusual' indicates an anagram.
'greek character's' becomes 'mus' (letter from the Greek alphabet).
'this' is an anagram of 'isth'.
'isth'+'mus'='ISTHMUS'
(Other definitions for isthmus that I've seen before include "Narrow strip of land between two larger ones" , "Narrow strip of land connecting two large masses" , "Narrow strip joining larger areas of land" , "This sum for a narrow piece of land" , "Narrow linking piece of land" .)