Without her, one would be callous and cruel (4)
I believe the answer is:
ruth
'without her one would' is the definition.
I can't judge whether this definition defines the answer.
'be callous and cruel' is the wordplay.
I cannot really see how this works, but
'be' could be 'rut' (rutting is a kind of being) and 'rut' is found in the answer.
A single letter 'h' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ruth that I've seen before include "Babe -, US baseball star" , "Babe -, US baseball player" , "Old word for pity, or Bible book" , "Biblical Book with woman's name - a pity" , "Girl's name and Old Testament Book" .)