Material of special kind (4)
I believe the answer is:
silk
'material' is the definition.
(silk is a kind of material)
'special kind' is the wordplay.
'special' becomes 's' (common abbreviation as in Special Air Service).
'kind' becomes 'ilk' (ilk is a kind of kind**).
's'+'ilk'='SILK'
'of' acts as a link.
(Other definitions for silk that I've seen before include "it's taken by QC" , "legal advisor" , "Fine fabric -- senior barrister" , "Lustrous fibre" , "Senior lawyer; type of worm" .)