Neckwear could be junk, mostly (3)
I believe the answer is:
boa
'neckwear' is the definition.
(boa can mean a scarf)
'junk mostly' is the wordplay.
'junk' becomes 'boat' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'boat' with its last letter taken away is 'BOA'.
'could be' acts as a link.
(Other definitions for boa that I've seen before include "Fluffy scarf of feathers" , "One with scaly body" , "Large constrictor snake" , "... cold-blooded creature" , "S American snake that crushes its prey" .)