How the dealer struggles to be sincere (12)
I believe the answer is:
wholehearted
'be sincere' is the definition.
('wholehearted' can be similar in meaning to 'sincere')
'how the dealer struggles' is the wordplay.
'struggles' indicates anagramming the letters.
'how'+'the'+'dealer'='howthedealer'
'howthedealer' anagrammed gives 'WHOLEHEARTED'.
'to' acts as a link.
(Other definitions for wholehearted that I've seen before include "Earnest" , "Unqualified" , "With enthusiastic devotion" , "Passionate" , "Unreserved - affectionate" .)