Sentimental affection, mostly on what we know is low-key (4-6)
I believe the answer is:
'we know is low-key' is the definition.
I can't judge whether this defines the answer.
'sentimental affection mostly on what' is the wordplay.
'sentimental affection' becomes 'softspot' (I've seen this before).
'mostly' means to remove the last letter.
'on' is a charade indicator (letters next to each other).
'what' becomes 'ken' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'softspot' with its final letter taken away is 'softspo'.
Can you help me to learn more?
(Another definition for soft-spoken that I've seen is " Having a mild or gentle voice".)