Hardest part of the garden to cultivate (7)
I believe the answer is:
rockery
'hardest part' is the definition.
'rockery' can be an answer for 'part' (I have seen 'Part of garden' mean 'rockery' so perhaps 'part' could also mean 'rockery'). I am not certain of the 'hardest' bit.
'garden to cultivate' is the wordplay.
'garden' becomes 'rock garden' (rock garden is a kind of garden).
'to cultivate' becomes 'ery' (I can't explain this - if you can you should give a lot more credence to this answer).
'rock'+'ery'='ROCKERY'
'of the' is the link.
Can you help me to learn more?
(Other definitions for rockery that I've seen before include "Garden alpine area" , "Part of garden" , "Garden feature with alpines" .)