Mostly stop round circle of light (4)
I believe the answer is:
halo
'circle of light' is the definition.
(I've seen this before)
'mostly stop round' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'stop' becomes 'halt' (halt can mean to stop).
'round' becomes 'o' (o is a circular letter).
'halt' with its last letter removed is 'hal'.
'hal'+'o'='HALO'
(Other definitions for halo that I've seen before include "Sign of holiness" , "Angel's head-light" , "Corona" , "Glowing ring (around saint's head)" , "Saint's headlight?" .)