Milliner's product brought about passion to secure new headwear (7)
I believe the answer is:
hairnet
'headwear' is the definition.
(I know that hairnet is a type of headwear)
'milliner's product brought about passion to secure new' is the wordplay.
'milliner's product' becomes 'hat' (I've seen this in another clue).
'brought about' is an insertion indicator.
'passion' becomes 'ire' (ire is passionate anger).
'to secure' indicates putting letters inside (inserted letters are secured inside).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'ire' enclosing 'n' is 'irne'.
'hat' enclosing 'irne' is 'HAIRNET'.
(Other definitions for hairnet that I've seen before include "It keeps locks in place" , "It may keep locks" , "The rain (anag) - used after shampooing?" , "Tonsorial restraint" , "It protects locks" .)