Lid used as iron horse's cover (9)
I believe the answer is:
caparison
'horse's cover' is the definition.
(I've seen this before)
'lid used as iron' is the wordplay.
'lid' becomes 'cap' (I've seen this before).
'used' indicates anagramming the letters.
'as'+'iron'='asiron'
'asiron' is an anagram of 'arison'.
'cap'+'arison'='CAPARISON'
(Other definitions for caparison that I've seen before include "Cover for a horse" , "horse's cover" , "covering for the Shires?" , "sumptuous clothing" , "Decorative covering for a horse" .)