Nimbus - a touch of highlight, behold, encircling angel’s head (4)
I believe the answer is:
halo
'nimbus' is the definition.
(synonyms)
'a touch of highlight behold encircling angel's head' is the wordplay.
'a touch of' says to take the initial letters.
'behold' becomes 'lo' (archaic exclamation meaning 'look!').
'encircling' is an insertion indicator.
'head' says to take the initial letters.
The initial letter of 'highlight' is 'h'.
The initial letter of 'angels' is 'a'.
'h'+'lo'='hlo'
'hlo' going around 'a' is 'HALO'.
(Other definitions for halo that I've seen before include "Corona" , "Imaginary circle over saint's head" , "Aura round admired person" , "Crown" , "Circle of (holy) light" .)