Takes up arms and starts to repel enemy units (7)
I believe the answer is:
revolts
'takes up arms' is the definition.
Although both the answer and definition are verbs in their -s form, I can't see how they can define each other.
'starts to repel enemy units' is the wordplay.
'starts to' says to take the initial letters.
'units' becomes 'volts' (volt is a kind of unit).
The first letters of 'repel enemy' is 're'.
're'+'volts'='REVOLTS'
'and' is the link.
Can you help me to learn more?
(Other definitions for revolts that I've seen before include "Disgusts" , "Rebels" , "Acts of defiance" .)