Neckwear that's Siamese, by the sound of it (3)
I believe the answer is:
tie
'neckwear' is the definition.
(type of neckwear)
'siamese by the sound of it' is the wordplay.
'siamese' becomes 'Thai' (synonyms).
'by the sound of it' shows a homophone (sound like).
'thai' sounds like 'TIE'.
'that's' acts as a link.
(Other definitions for tie that I've seen before include "Draw for neckware" , "may make one hot under the collar" , "Neck-cloth" , "Get even" , "Draw - knot" .)