Standard Catholic ceremony on top of wrought iron (9)
I believe the answer is:
criterion
'standard' is the definition.
(I know that standard can be written as criterion)
'catholic ceremony on top of wrought iron' is the wordplay.
'catholic' becomes 'C' (abbreviation as in RC for Roman Catholic).
'ceremony' becomes 'rite' (rite is a kind of ceremony).
'on top' means one lot of letters go next to another (in a down clue, some letters go above others).
'of wrought' indicates anagramming the letters (I've seen 'wrought' mean this (I've seen this in other clues)).
'iron' is an anagram of 'rion'.
'c'+'rite'+'rion'='CRITERION'
(Other definitions for criterion that I've seen before include "Judging standard" , "London theatre?" , "Standard, reference point" , "Standard; model" , "Benchmark - standard" .)