Unprofitable land, space that's taken up (4)
I believe the answer is:
moor
'unprofitable land' is the definition.
'moor' can be an answer for 'land' (moor is a kind of land). I am not sure about the 'unprofitable' bit.
'space that's taken up' is the wordplay.
'space' becomes 'room' (enough space or room to do something).
'that's taken up' says the letters should be written backwards.
'room' back-to-front is 'MOOR'.
Can you help me to learn more?
(Other definitions for moor that I've seen before include "'On Ilka - - baht'at'" , "The Battle of Marston - -" , "Upland tract - secure boat" , "Open expanse covered with heather and bracken" , "Othello, perhaps" .)