Call about all decentralised glory (4)
I believe the answer is:
halo
'glory' is the definition.
(I know that glory can be written as halo)
'call about all decentralised' is the wordplay.
'call' becomes 'ho' (historical exclamation used to attract attention).
'about' means one lot of letters goes inside another.
'decentralised' suggests removing the centre (the centre of the word is removed).
'all' with its middle taken out is 'al'.
'ho' going around 'al' is 'HALO'.
(Other definitions for halo that I've seen before include "Circle over a saint" , "Aureole" , "Circle of light around the moon" , "Glowing ring (around saint's head)" , "that could make one light-headed?" .)