Arrive with opposition extremists and attack (4,4)

'attack' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'arrive with opposition extremists' is the wordplay.
'arrive' becomes 'turnup' (I've seen this in another clue).
'with' says to put letters next to each other.
'extremists' suggests removing the centre (letters on the extreme ends of the word).
'opposition' with its middle removed is 'on'.
'turnup'+'on'='TURN UPON'

'and' acts as a link.

Can you help me to learn more?