Outrageously flatter the man at the top -- being sincere? (9)
I believe the answer is:
heartfelt
'sincere?' is the definition.
('heartfelt' can be similar in meaning to 'sincere')
'outrageously flatter the man at the top' is the wordplay.
'outrageously' indicates anagramming the letters (I've seen this in other clues).
'the man' becomes 'he'.
'at the top' means one lot of letters go next to another.
'flatter' is an anagram of 'artfelt'.
'artfelt' put after 'he' is 'HEARTFELT'.
'being' acts as a link.
(Other definitions for heartfelt that I've seen before include "Deeply sincere" , "Profound" , "With sincere passion" , "Kindly" , "unfeigned sincerity" .)