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' says to put letters next to each other.
'quit' becomes 'left' (I've seen this before).
'regressing' shows that the letters should be reversed in order (to regress is to go backwards).
'tons' becomes 't'.
'left' back-to-front is '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" .)