It's a bit of a job to walk where there are many thorns (7)
I believe the answer is:
bramble
'where there are many thorns' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'it's a bit of a job to walk' is the wordplay.
'it's a bit of a job' becomes 'b' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to walk' becomes 'ramble' (ramble is a kind of walk).
'b'+'ramble'='BRAMBLE'
Can you help me to learn more?
(Other definitions for bramble that I've seen before include "Thorny bush" , "Prickly plant yielding blackberries" , "Dog rose" , "Source of fruit" , "Blackberry shrub" .)