Let father reform, and be sincere! (9)
I believe the answer is:
heartfelt
'be sincere' is the definition.
('heartfelt' can be similar in meaning to 'sincere')
'let father reform' is the wordplay.
'reform' is an anagram indicator.
'let'+'father'='letfather'
'letfather' anagrammed gives 'HEARTFELT'.
'and' acts as a link.
(Other definitions for heartfelt that I've seen before include "Most fervent in cardiac way" , "Kindly" , "Profound" , "Deeply sincere" , "Sincerely intended with strong feeling" .)