Iron on unknown head wear (3)
I believe the answer is:
fez
'head wear' is the definition.
'fez' can be an answer for 'wear' (fez is a kind of wear). I am not certain of the 'head' bit.
'iron on unknown' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'unknown' becomes 'z' (algebra).
'fe'+'z'='FEZ'
Can you help me to learn more?
(Other definitions for fez that I've seen before include "Brimless conical cap" , "Felt hat" , "Tasselled hat" , "Hat - Moroccan city" , "Red hat with black tassel worn by some Muslims" .)