Gutted those Republican donors had to switch allegiance (12)
I believe the answer is:
tergiversate
'to switch allegiance' is the definition.
I can't judge whether this defines the answer.
'gutted those republican donors had' is the wordplay.
'gutted' suggests removing the centre (to gut something is to remove its insides).
'republican' becomes 'r' (abbreviation).
'donors' becomes 'givers' ('giver' can be a synonym of 'donor').
'had' becomes 'ate' (e.g. I had an ice cream).
'those' with its middle removed is 'te'.
'te'+'r'+'givers'+'ate'='TERGIVERSATE'
Can you help me to learn more?
(Other definitions for tergiversate that I've seen before include "desert" , "Shilly-shally" , "Equivocate" , "avoid the issue" .)