Initially slippery and smooth (4)
I believe the answer is:
sand
'smooth' is the definition.
(sanding is a kind of smoothing)
'initially slippery and' is the wordplay.
'initially' suggests taking the first letters.
The first letter of 'slippery' is 's'.
's'+'and'='SAND'
(Other definitions for sand that I've seen before include "Loose grains" , "Dune constituent" , "Desert grit" , "Hourglass contents" , "Very fine grains of rock, as on beach" .)