Luscious oranges and melons primarily growing in this? (4)
I believe the answer is:
loam
'growing in this?' is the definition.
(loam is a type of soil)
'luscious oranges and melons primarily' is the wordplay.
'primarily' says to take the initial letters.
The first letters of 'luscious oranges and melons' is 'LOAM'.
(Other definitions for loam that I've seen before include "Type of soil" , "Rich soil, good for growing" , "A rich fertile soil" , "Rich soil consisting of a mixture of sand, clay and decaying organic materials" , "Rich soil with decayed vegetable matter" .)