P, not L, is a kind-hearted soul (5)
I believe the answer is:
softy
'p not l' is the definition.
I can't tell whether this definition defines the answer.
'kind-hearted soul' is the wordplay.
'kind' becomes 'soft' (associated in meaning).
'hearted soul' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'soft'+'y'='SOFTY'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for softy that I've seen before include "Sentimental person" , "Weak person" , "Gentle sort" .)