Insincerity of thank you drowned in sentiment (11)
I believe the answer is:
affectation
'insincerity' is the definition.
(thesaurus)
'thank you drowned in sentiment' is the wordplay.
'thank you' becomes 'ta'.
'drowned in' indicates putting letters inside (I've seen this in other clues).
'sentiment' becomes 'affection'.
'ta' placed into 'affection' is 'AFFECTATION'.
'of' acts as a link.
(Other definitions for affectation that I've seen before include "retence" , "Deliberate pretence" , "unnatural appearance" , "Show evidence of" .)