Tidy mess made by Englishman in Australia, as it were (3,2)
I believe the answer is:
mop up
'tidy' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Maybe you can see an association between them that I don't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for mop up that I've seen before include "Finish a task, clear away remnants" , "Kill (enemy stragglers)" , "polish off" , "take care of" , "Clear away, dispose of" .)