By the sound of it, yarns for formal wear (5)
I believe the answer is:
tails
'formal wear' is the definition.
(as in tail-suits)
'by the sound of it yarns' is the wordplay.
'by the sound of it' shows a homophone (sound like).
'yarns' becomes 'tales' ('tale' can be a synonym of 'yarn').
'tales' sounds like 'TAILS'.
'for' acts as a link.
(Other definitions for tails that I've seen before include "Follows closely" , "Evening dress; coin toss call" , "the back of bears - bulls too" , "Coin-tossing call" , "Morning coat" .)