Door where walls in extension go (5)
I believe the answer is:
entry
'door' is the definition.
(synonyms)
'walls in extension go' is the wordplay.
'walls in' says to hollow out the word (remove centre letters).
'go' becomes 'try' (both can mean an attempt).
'extension' with its middle taken out is 'en'.
'en'+'try'='ENTRY'
'where' is the link.
(Other definitions for entry that I've seen before include "Doorway" , "Access point" , "Way in; item in list" , "Admittance" , "List of competitors" .)