Old Irish liberal in bizarre sect (5)
I believe the answer is:
celts
'old irish' is the definition.
(I've seen this in another clue)
'liberal in bizarre sect' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'in' means one lot of letters goes inside another.
'bizarre' indicates an anagram.
'sect' with letters rearranged gives 'cets'.
'l' placed inside 'cets' is 'CELTS'.
(Other definitions for celts that I've seen before include "Pre-Roman inhabitants of much of Europe" , "Ancient Irish, Scots or Welsh" , "Ancient people of Europe" .)