Gum or nuts one started chewing (6)
I believe the answer is:
mastic
'gum' is the definition.
(mastic is a resin also called arabian gum)
'nuts one started chewing' is the wordplay.
'nuts' becomes 'mast' (mast is the nuts of forest trees).
'one' becomes 'i' (Roman numeral).
'started' suggests taking the first letters.
The first letter of 'chewing' is 'c'.
'mast'+'i'+'c'='MASTIC'
'or' is the link.
(Other definitions for mastic that I've seen before include "Substance applied to joints" , "Aromatic resin used to make varnish" , "Mac, sit and produce a gum resin" , "Putty-like cement used as adhesive or filler" , "Resin used in varnishes" .)