Sincere to try and quit, regressing tons (9)
I believe the answer is:
heartfelt
'sincere' is the definition.
(similar in meaning)
'try and quit regressing tons' is the wordplay.
'try' becomes 'hear' (as in to try someone in court or hear their case).
'and' means one lot of letters go next to another.
'quit' becomes 'left' (I've seen this before).
'regressing' is a reversal indicator (to regress is to go backwards).
'tons' becomes 't'.
'left' written backwards gives 'tfel'.
'hear'+'tfel'+'t'='HEARTFELT'
'to' is the link.
(Other definitions for heartfelt that I've seen before include "Let father have deep and earnest kind of emotion (9)" , "Deeply sincere" , "Kindly" , "With sincere passion" , "unfeigned sincerity" .)