Got it from group losing the head with politician over the Celtic goddess from summer long ago (8)
I believe the answer is:
obtained
'got' is the definition.
('obtain' can be a synonym of 'get')
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for obtained that I've seen before include "Into a bed (anag.)" , "Got something" , "Got, procured" , "Acquired" , "Came into possession of" .)