Reeds cultivated beside a large body of water (3,3)
I believe the answer is:
red sea
'body of water' is the definition.
(the Red Sea is a body of water)
'reeds cultivated beside a large' is the wordplay.
I cannot really see how this works, but
an anagram of 'reeds' is 'redse' which is present in the answer.
'a' is within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for red sea that I've seen before include "Long narrow stretch of water between Africa and Arabia" , "Water south of Sinai" , "It parted for Moses and the Israelite (3,3)" , "It parted for Moses and his followers" , "It divides Arabia/Africa" .)