Great deal on land makes a killing (8)
I believe the answer is:
massacre
'a killing' is the definition.
(massacre is a kind of killing)
'great deal on land' is the wordplay.
'great deal' becomes 'mass' (synonyms).
'on land' becomes 'acre' (I've seen this before. I am not sure about the 'on' bit.).
'mass'+'acre'='MASSACRE'
'makes' is the link.
Can you help me to learn more?
(Other definitions for massacre that I've seen before include "Complete defeat" , "Butchery" , "butcher" , "Carnage" , "Yours and mine" .)