Whence to come down to earth in port (6)
I believe the answer is:
heaven
'whence to' is the definition.
I can't tell whether this defines the answer.
'come down to earth in port' is the wordplay.
'come down to earth' becomes 'e' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'port' becomes 'haven' (haven is a kind of port).
'e' placed within 'haven' is 'HEAVEN'.
Can you help me to learn more?
(Other definitions for heaven that I've seen before include "Abode of the blessed" , "The abode of God in many religions" , "State of bliss" , "Ideal place" , "Sky - paradise" .)