Slippery slope's end is submerged by Serpentine (6)
I believe the answer is:
sneaky
'slippery' is the definition.
(thesaurus)
'slope's end is submerged by serpentine' is the wordplay.
'end' suggests the final letters.
'is submerged by' indicates putting letters inside.
'serpentine' becomes 'snaky' (I've seen this before).
The last letter of 'slope' is 'e'.
'e' going into 'snaky' is 'SNEAKY'.
(Other definitions for sneaky that I've seen before include "Cunning" , "Furtive, lacking candour" , "Furtive, underhand" , "Deceitful, underhand" , "Being furtive or underhanded" .)