Felt is used, used to emphasise something (6)
I believe the answer is:
itself
'used to emphasise something' is the definition.
(reflexive pronoun emphasising mention of an object)
'felt is used' is the wordplay.
'used' indicates anagramming the letters.
'felt'+'is'='feltis'
'feltis' is an anagram of 'ITSELF'.
(Other definitions for itself that I've seen before include "The thing, I stress" , "Pronoun used to reflect the object or for emphasis" , "Third-person reflexive pronoun" , "The thing, reflexively" , "alone by this" .)