They’re in behind tooth, primarily cutting gum with one (6)
I believe the answer is:
glutei
'they're in behind' is the definition.
(plural of gluteus, muscles in one's behind)
'tooth primarily cutting gum with one' is the wordplay.
'primarily' indicates taking the first letters.
'cutting' indicates putting letters inside (some letters cut their way into a word).
'gum' becomes 'glue' ('glue' can be a synonym of 'gum'**).
'with' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
The initial letter of 'tooth' is 't'.
't' going into 'glue' is 'glute'.
'glute'+'i'='GLUTEI'
(Other definitions for glutei that I've seen before include "behind?" , "muscles" .)