Fastening hardly by one's ear? (4)
I believe the answer is:
knot
'fastening' is the definition.
(knot is a kind of fastening)
'hardly by one's ear?' is the wordplay.
'hardly' becomes 'not' (I've seen this in another clue).
'by one's ear?' shows a homophone (sound like).
'not' sounds like 'KNOT'.
(Other definitions for knot that I've seen before include "Tie; group" , "Entangle" , "Unit of speed in water" , "Fastener - bird - unit of speed at sea" , "Reef or granny, say" .)