Fellow's outer door starts to open after knock (3)
I believe the answer is:
oak
'fellow's outer door' is the definition.
I can't tell whether this definition defines the answer.
'starts to open after knock' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'open after knock' is 'OAK'.
Can you help me to learn more?
(Other definitions for oak that I've seen before include "Its seed is the acorn" , "Common forest tree" , "Acorn-bearing tree" , "Hardwood tree" , "Strong hardwood" .)