Sign attached to back of iron gate (7)
I believe the answer is:
postern
'gate' is the definition.
(postern is a kind of gate)
'sign attached to back of iron' is the wordplay.
'sign' becomes 'post' (post is a kind of sign).
'attached to' means one lot of letters go next to another.
'back of iron' becomes 'ern' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'post'+'ern'='POSTERN'
(Other definitions for postern that I've seen before include "Back door eg of castle" , "Back or side entrance" , "private entrance" , "Rear entrance" , "Small gate in the rear of a fort or castle" .)