Iron lady's soft hat (6)
I believe the answer is:
fedora
'soft hat' is the definition.
(I've seen this in another clue)
'iron lady's' is the wordplay.
'iron' becomes 'Fe' (chemical symbol for iron).
'lady' becomes 'dora' ().
'fe'+'dora'='FEDORA'
(Other definitions for fedora that I've seen before include "Felt hat, dented lengthwise" , "Felt hat with a creased crown" , "O, Fred, a hat becomes you" , "Felt hat much loved by 1930s gangsters" , "Type of hat" .)