Sort of art in leading couple abandoning TU principles, on paper (13)
I believe the answer is:
expressionism
'sort of art' is the definition.
'principles on paper' is the wordplay.
'principles' becomes 'ionism' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'paper' becomes 'express' (The Daily Express newspaper).
'ionism' after 'express' is 'EXPRESSIONISM'.
'in leading couple abandoning tu' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for expressionism that I've seen before include "Art movement" , "art of emotion" , "Style of painting" , "Artistic style" , "Art school" .)