Extra tie David knotted (8)
I believe the answer is:
additive
'extra' is the definition.
'tie david knotted' is the wordplay.
'knotted' indicates an anagram.
'tie'+'david'='tiedavid'
'tiedavid' is an anagram of 'ADDITIVE'.
(Other definitions for additive that I've seen before include "Substance stirred in to colour or flavour food" , "flavouring perhaps" , "Something put into food" , "Substance put into food" , "Eg, a colorant or preservative in foods" .)