Cover uncultivated land in middle of desert (7)
I believe the answer is:
'cover' is the definition.
(to sheathe something is to cover it)
'uncultivated land in middle of desert' is the wordplay.
'uncultivated land' becomes 'heath' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'middle' becomes 'see' (see is a kind of middle).
'of desert' says to hollow out the word (remove centre letters) (I've seen 'deserted' mean this).
'see' with its middle taken out is 'se'.
'heath' going inside 'se' is 'SHEATHE'.
(Other definitions for sheathe that I've seen before include "Encase in protection" , "stow away safely" , "Put into scabbard" , "Put (a sword) into case" , "Put away (a sword)" .)