Old Irish student in strange sect (5)
I believe the answer is:
celts
'old irish' is the definition.
(I've seen this in another clue)
'student in strange sect' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'in' indicates putting letters inside.
'strange' indicates anagramming the letters.
'sect' with letters rearranged gives 'cets'.
'l' put into 'cets' is 'CELTS'.
(Other definitions for celts that I've seen before include "Ancient Irish, Scots or Welsh" , "Pre-Roman inhabitants of much of Europe" , "Ancient people of Europe" .)