Iron Lady’s felt hat (6)
I believe the answer is:
fedora
'felt hat' is the definition.
('fedora' can be a synonym of 'felt hat')
'iron lady's' is the wordplay.
'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 "Style of hat" , "Headgear" , "Type of hat" , "Felt hat dented lengthways" , "O, Fred, a hat becomes you" .)