Professor got irregular but persisted (7,2)
I believe the answer is:
dragged on
'persisted' is the definition.
The definition and answer can be both to do with fundamental existence as well as being past participle verbs.
Maybe there's a link between them I don't understand?
'professor got irregular' is the wordplay.
'professor' becomes 'don' (term for a university professor).
'got' indicates putting letters inside.
'irregular' becomes 'ragged' (I've seen this in another clue).
'don' going around 'ragged' is 'DRAGGED ON'.
'but' is the link.
Can you help me to learn more?
(Other definitions for dragged on that I've seen before include "continued tediously" , "failed to finish" .)