Cease attacking and allow a fallen opponent to rise (3,2)
I believe the answer is:
let up
'cease' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Perhaps you can see a link between them that I don't see?
'attacking and allow a fallen opponent to rise' is the wordplay.
I cannot really see how this works, but
'allow' could be 'let' (synonyms) and 'let' is found in the answer.
'rise' could be 'up' (as a noun) and 'up' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for let up that I've seen before include "Pause in intensity" , "Relaxation of effort, reduction in intensity" , "Relax one's efforts" , "Die away, ease off" , "Relent, relax" .)