Single sun ray, it flickers (8)
I believe the answer is:
solitary
'single' is the definition.
(similar in meaning)
'sun ray it flickers' is the wordplay.
'sun' becomes 'sol' (Latin for 'sun').
'flickers' indicates an anagram (I've seen 'flickering' mean this (I've seen this in other clues)).
'ray'+'it'='rayit'
'rayit' is an anagram of 'itary'.
'sol'+'itary'='SOLITARY'
(Other definitions for solitary that I've seen before include "friendless" , "unique" , "Being the only one" , "Recluse" , "not a Companion" .)