Milliner's stock (4)
I believe the answer is:
hats
'milliner's stock' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hats that I've seen before include "Headgear" , "Milliner's stock" , "Headdresses" , ""Porkpie" and "pillbox" attire" , "Headwear" .)