Everything in neckwear matches (7)
I believe the answer is:
tallies
'matches' is the definition.
(I know this)
'everything in neckwear' is the wordplay.
'everything' becomes 'all' (all things).
'in' is an insertion indicator.
'neckwear' becomes 'ties' (I've seen this in another clue).
'all' inserted inside 'ties' is 'TALLIES'.
(Other definitions for tallies that I've seen before include "Agrees; scores" , "Recordings" , "Reckons debt or score" , "Matches, corresponds" , "Records" .)