Material of singular kind, full of witticisms (4,4)
I believe the answer is:
spun silk
'material' is the definition.
(I know that spun silk is a type of yarn)
'singular kind full of witticisms' is the wordplay.
'singular' becomes 's' (abbreviation in grammar).
'kind' becomes 'ilk' (ilk can mean a type or kind).
'full of' indicates putting letters inside.
'witticisms' becomes 'puns' (pun is a kind of witticism).
's'+'ilk'='silk'
'silk' placed around 'puns' is 'SPUN SILK'.
'of' acts as a link.
(Other definitions for spun silk that I've seen before include "Cheap fabric" , "Cheap material made from waste fibres" , "Yarn" .)