Be keen to pass on writing with pen (8)
I believe the answer is:
longhand
'with pen' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'be keen to pass on' is the wordplay.
'be keen' becomes 'long' (I can't justify this - if you can you should give a lot more credence to this answer).
'to pass on' becomes 'hand' ('hand' can be a synonym of 'pass on').
'long'+'hand'='LONGHAND'
'writing' is the link.
Can you help me to learn more?
(Other definitions for longhand that I've seen before include "script" , "Joined-up writing" , "Having words written out in full by hand" , "It's normally written" , "Ordinary writing of manual origin" .)