Old hat said to irritate hard patch of skin (7)
I believe the answer is:
tricorn
'old hat' is the definition.
(I've seen this before)
'said to irritate hard patch of skin' is the wordplay.
'said' indicates a 'sounds like' (homophone) clue.
'to irritate' becomes 'try' (I've seen this before).
'hard patch of skin' becomes 'corn' (hard skin on a foot).
'try' is a homophone of 'tri'.
'tri'+'corn'='TRICORN'
(Other definitions for tricorn that I've seen before include "head wear" , "headgear" , "old hat" , "Cocked hat with brim turned up on three sides" , "Hat with upturned brim" .)