Not mentioned by granny, perhaps (4)
I believe the answer is:
knot
'granny perhaps' is the definition.
(granny is a kind of knot)
'not mentioned' is the wordplay.
'mentioned' shows a homophone (sound like).
'not' sounds like 'KNOT'.
'by' acts as a link.
(Other definitions for knot that I've seen before include "one may find in wood" , "Sort of garden" , "Unit of speed used by ships" , "1 (nautical) mph" , "Fastening - unit of speed" .)