Apostle endlessly touring dead flat land (7)
I believe the answer is:
plateau
'flat land' is the definition.
(plateau can mean a piece of flat land)
'apostle endlessly touring dead' is the wordplay.
'apostle' becomes 'paul' (St Paul is an example).
'endlessly' means to remove the last letter.
'touring' is an insertion indicator (touring can mean going round).
'dead' becomes 'late' (similar in meaning).
'paul' with its last letter removed is 'pau'.
'pau' going around 'late' is 'PLATEAU'.
(Other definitions for plateau that I've seen before include "one has no inclination" , "tray that is ornamented" , "High, level tract of land" , "Levelling off" , "Flat highland" .)