Merciless-sounding needle (6)
I believe the answer is:
crewel
'needle' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?
'merciless-sounding' is the wordplay.
'merciless' becomes 'cruel' (I've seen this before).
'sounding' indicates a 'sounds like' (homophone) clue.
'cruel' sounds like 'CREWEL'.
Can you help me to learn more?
(Other definitions for crewel that I've seen before include "Fine worsted yarn" , "Twisted yarn for tapestry or embroidery" .)