Bring up millions and prepare for war (5)
I believe the answer is:
rearm
'prepare for war' is the definition.
'bring up millions' is the wordplay.
'bring up' becomes 'rear' (rearing is a kind of bringing up).
'millions' becomes 'm' (abbreviation).
'rear'+'m'='REARM'
'and' acts as a link.
(Other definitions for rearm that I've seen before include "Strengthen one's defences" , "fit again" , "Provide with a new supply of weapons" , "prepare for another war" , "Acquire new or better weapons" .)