Fasten thus to the ear (3)
I believe the answer is:
sew
'fasten' is the definition.
(sewing is a kind of fastening)
'thus to the ear' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'to the ear' indicates a 'sounds like' (homophone) clue.
'so' is a homophone of 'SEW'.
(Other definitions for sew that I've seen before include "Fasten with needle and thread" , "Work with needle and thread" , "Stitch with needle and thread" , "Make stitches" , "Fasten with stitches" .)