The Iron Lady's hat (6)
I believe the answer is:
fedora
'hat' is the definition.
(fedora is a kind of hat)
'the iron lady's' is the wordplay.
'the iron' becomes 'fe' (Fe is the chemical symbol for iron).
'lady' becomes 'dora' (**).
'fe'+'dora'='FEDORA'
(Other definitions for fedora that I've seen before include "Felt hat dented lengthways" , "Cover for head" , "top fashion" , "Style of hat" , "Headgear" .)