Prepare anew to get a back number (5)
I believe the answer is:
rearm
'prepare anew' is the definition.
I can't judge whether this definition defines the answer.
'get a back number' is the wordplay.
'get' is a charade indicator (letters next to each other).
'a back' becomes 'rear' (something's rear is its back).
'number' becomes 'm' (M is an example).
'rear' put next to 'm' is 'REARM'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for rearm that I've seen before include "Acquire new or better weapons" , "Increase weaponry" , "Strengthen one's defences" , "prepare to fight again" , "Hand new weapons" .)