Deb's smile, sadly, is fake (9)
I believe the answer is:
dissemble
'is fake' is the definition.
(thesaurus)
'deb's smile sadly' is the wordplay.
'sadly' indicates anagramming the letters.
'debs'+'smile'='debssmile'
'debssmile' is an anagram of 'DISSEMBLE'.
(Other definitions for dissemble that I've seen before include "Conceal or disguise feelings or motives" , "Disguise one's true feelings" , "Hide true motives" , "Hide one's true feelings" , "Conceal one's true feelings" .)