I am leaving old king a fertile piece of land (7)
I believe the answer is:
orchard
'a fertile piece of land' is the definition.
(land for growing fruit)
'i am leaving old king' is the wordplay.
'am leaving' suggests deleting specific letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'king' becomes 'richard' (Richard I or Richard II).
'o'+'richard'='orichard'
'orichard' with 'i' taken away is 'ORCHARD'.
(Other definitions for orchard that I've seen before include "Fruit tree plantation" , "Area set aside to grow fruit" , "Where fruit trees are grown" , "Trees bearing apples perhaps" , "Fruit grove" .)