Iron rod upon a hat (6)
I believe the answer is:
fedora
'hat' is the definition.
(fedora is a kind of hat)
'iron rod upon a' is the wordplay.
'iron' becomes 'fe' ('Fe' can be a synonym of 'iron').
'rod' becomes 'dor' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'upon' means one lot of letters go next to another.
'fe'+'dor'+'a'='FEDORA'
(Other definitions for fedora that I've seen before include "Brimmed hat with creased crown" , "Cover for head" , "Type of hat" , "Style of hat" , "top fashion" .)