The chap to flatter, perhaps, but it must be sincere (9)
I believe the answer is:
heartfelt
'be sincere' is the definition.
(similar in meaning)
'the chap to flatter perhaps' is the wordplay.
'the chap' becomes 'he'.
'to' is a charade indicator (letters next to each other).
'perhaps' indicates an anagram.
'flatter' is an anagram of 'artfelt'.
'he'+'artfelt'='HEARTFELT'
'but it must' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for heartfelt that I've seen before include "Let father have deep and earnest kind of emotion (9)" , "Deeply sincere" , "Profound" , "Kindly" , "Sincerely intended with strong feeling" .)