Carpet Simon had changed (8)
I believe the answer is:
admonish
'carpet' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how they can define each other.
'simon had changed' is the wordplay.
'changed' indicates an anagram.
'simon'+'had'='simonhad'
'simonhad' is an anagram of 'ADMONISH'.
Can you help me to learn more?
(Other definitions for admonish that I've seen before include "Reprove, warn" , "Tell off" , "Scold, wag the finger at" , "Chide, reprove" , "Firmly reprimand" .)