Sincere father let off (9)
I believe the answer is:
heartfelt
'sincere' is the definition.
(similar in meaning)
'father let off' is the wordplay.
'off' is an anagram indicator.
'father'+'let'='fatherlet'
'fatherlet' anagrammed gives 'HEARTFELT'.
(Other definitions for heartfelt that I've seen before include "Deeply sincere" , "Kindly" , "Profound" , "Let father have deep and earnest kind of emotion (9)" , "With sincere passion" .)