Theme work penned by Mark that remains unfinished (5)
I believe the answer is:
topic
'theme work penned by mark that remains unfinished' is the definition.
'topic' can be an answer for 'theme' (synonyms). I'm not sure about the remainder of the definition.
'theme work penned by' is the wordplay.
'theme' becomes 'tic' (I can't justify this - if you can you should give a lot more credence to this answer).
'work' becomes 'op' (common abbreviation for 'opus').
'penned by' indicates putting letters inside.
'tic' going around 'op' is 'TOPIC'.
Can you help me to learn more?
(Other definitions for topic that I've seen before include "Subject (of a talk, etc)" , "Subject (of, eg, speech)" , "Something to discuss" , "That may be debatable" , "Theme of discourse" .)