Awful creep, about ten, behaving insincerely (8)
I believe the answer is:
pretence
'behaving insincerely' is the definition.
(pretence is false or pretended behaviour)
'awful creep about ten' is the wordplay.
'awful' is an anagram indicator.
'about' indicates putting letters inside.
'creep' with letters rearranged gives 'prece'.
'prece' enclosing 'ten' is 'PRETENCE'.
(Other definitions for pretence that I've seen before include "Erect pen (anag) - charade" , "Act of make-believe" , "Ten creep (anag.)" , "Make-believe, or false claim to merit" , "it can't be true!" .)