Attack a Democrat in comic vein (6)
I believe the answer is:
invade
'attack' is the definition.
(invading is a kind of attacking)
'a democrat in comic vein' is the wordplay.
'democrat' becomes 'd'.
'in' means one lot of letters goes inside another.
'comic' indicates an anagram.
'a'+'d'='ad'
'vein' is an anagram of 'inve'.
'ad' going within 'inve' is 'INVADE'.
(Other definitions for invade that I've seen before include "Overrun" , "Violate" , "Enter illegally" , "Enter a country by force with an army" , "Penetrate other country with armed forces" .)