The dog follows closely but reduces (8)
I believe the answer is:
'reduces' is the definition.
The answer and definition can be both related to change as well as being verbs in their -s form.
Perhaps you can see an association between them that I can't see?
'the dog follows closely' is the wordplay.
'the dog' becomes 'cur' (both can mean a despicable person).
'follows closely' becomes 'tails' (I've seen this in another clue).
'but' acts as a link.
Can you help me to learn more?
(Other definitions for curtails that I've seen before include "Shortens" , "Places restrictions on" , "Cuts short, reduces" .)