It joins in operations (6)
I believe the answer is:
'it joins in operations' is the definition.
The definition suggests a singular noun which matches the answer.
This is the entire clue.
Can you help me to learn more?
(Other definitions for suture that I've seen before include "Surgical stitch in uterus" , "Medical stitch" , "Material for sewing a wound" , "part of the healing process?" , "Thread for sewing up wound" .)