Unctuous and cunning when wife’s over (4)
I believe the answer is:
oily
'unctuous' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
(Other definitions for oily that I've seen before include "Greasy - obsequious" , "Viscous" , "Smooth; like lubricant" , "covered in grease" , "Smooth, ingratiating" .)