A thin peg, originally designed, that's pushed into hair (6)
I believe the answer is:
hatpin
'that's pushed into hair' is the definition.
(a hatpin pins a hat to the wearer's hair)
'a thin peg originally designed' is the wordplay.
'originally' says to take the initial letters.
'designed' indicates an anagram (designed with the letters).
The first letter of 'peg' is 'p'.
'a'+'thin'+'p'='athinp'
'athinp' is an anagram of 'HATPIN'.
(Other definitions for hatpin that I've seen before include "Bonnet fixer" , "Fastener for millinery" , "Fastener for headgear" , "Headgear fixer" .)