Mocking metal barrier (7)
I believe the answer is:
railing
This clue is a double definition.
'mocking' is the first definition.
(thesaurus)
'metal barrier' is the second definition.
(I've seen this before)
(Other definitions for railing that I've seen before include "Barrier, balustrade" , "Fence of metal bars" , "Gin lair (anag.)" , "Complaining - fence" , "Annoying" .)