Gentleness of old chap, sort of fat and due for treatment (10)
I believe the answer is:
mansuetude
'gentleness' is the definition.
(I've seen this before)
'old chap sort of fat and due for treatment' is the wordplay.
'old chap' becomes 'man' (I've seen this before. I am not sure about the 'old' bit.).
'sort of fat' becomes 'suet'.
'and' is a charade indicator (letters next to each other).
'for treatment' is an anagram indicator (I've seen this in other clues).
'due' anagrammed gives 'ude'.
'man'+'suet'+'ude'='MANSUETUDE'
'of' acts as a link.
Can you help me to learn more?
(Another definition for mansuetude that I've seen is " Gentleness".)