Scrap thin broken part of helmet (9)
I believe the answer is:
chinstrap
'part of helmet' is the definition.
(I know that chinstrap is part of helmet)
'scrap thin broken' is the wordplay.
'broken' indicates anagramming the letters.
'scrap'+'thin'='scrapthin'
'scrapthin' is an anagram of 'CHINSTRAP'.
(Other definitions for chinstrap that I've seen before include "Thong to hold on helmet" , "This secures cover for head" , "Retainer" , "Bit of helmet" , "securing device" .)