Picture rodent eating one beyond harbour (8)
I believe the answer is:
portrait
'picture' is the definition.
(portrait is a type of picture)
'rodent eating one beyond harbour' is the wordplay.
'rodent eating' becomes 'rait' (I can't explain this - if you can you should give a lot more credence to this answer).
'one beyond' means one lot of letters go next to another.
'harbour' becomes 'port' (I've seen this before).
'rait' put after 'port' is 'PORTRAIT'.
Can you help me to learn more?
(Other definitions for portrait that I've seen before include "Image of a person" , "Type of painting" , "Artistic representation (of a person)" , "(Person's) painting" , "Picture of person" .)