Unite extremists to do what’s necessary financially (4,4,4)
I believe the answer is:
make ends meet
This clue is a double definition.
'unite extremists' is the first definition.
(I've seen this before)
'do what's necessary financially' is the second definition.
(to make ends meet is to economise)
'to' is the link.
(Other definitions for make ends meet that I've seen before include "The Channel Tunnel consortium had to" , "Live within one's means" , "Just about manage" , "stay solvent" , "Live within one's income" .)