Obtain new weapons and a back number (5)
I believe the answer is:
rearm
'obtain new weapons' is the definition.
I can't tell whether this definition defines the answer.
'a back number' is the wordplay.
'a back' becomes 'rear' (something's rear is its back).
'number' becomes 'm' (M is an example).
'rear'+'m'='REARM'
'and' is the link.
Can you help me to learn more?
(Other definitions for rearm that I've seen before include "Equip with new or better weapons" , "Resupply with weapons" , "Provide with a new supply of weapons" , "Up the military budget?" , "Strengthen one's defences" .)