Loops put round excellent line in barriers (8)
I believe the answer is:
railings
'barriers' is the definition.
(railing is a kind of barrier)
'loops put round excellent line' is the wordplay.
'loops' becomes 'rings' (I am not sure about this - if you are sure you should believe this answer much more).
'put round' means one lot of letters goes inside another.
'excellent' becomes 'ai' (resembles 'Ar', old-fashioned term for excellent).
'line' becomes 'l' (used when specifying particular lines from a poem).
'ai'+'l'='ail'
'rings' enclosing 'ail' is 'RAILINGS'.
'in' is the link.
Can you help me to learn more?
(Other definitions for railings that I've seen before include "Iron bars set into protective fence" , "Fences of upright metal bars, often around parks" , "Protective barriers" , "bitter complaints" , "Metal guards" .)