Opening to enormous pit in garden (4)
I believe the answer is:
eden
'garden' is the definition.
(I know that Eden is an garden)
'opening to enormous pit' is the wordplay.
'opening to' suggests taking the first letters.
'pit' becomes 'den' (I am not sure about this - if you are sure you should believe this answer much more).
The initial letter of 'enormous' is 'e'.
'e'+'den'='EDEN'
'in' is the link.
Can you help me to learn more?
(Other definitions for eden that I've seen before include "Adam and Eve's garden" , "Tory Prime Minister, 1955-57" , "Perfect place" , "The near-perfect garden" , "Place of unspoilt beauty" .)