Lingers on a topic in which one has an abiding interest? (6)
I believe the answer is:
dwells
'lingers' is the definition.
The definition and answer can be both to do with fundamental existence as well as being verbs in their -s form.
Perhaps there's an association between them I don't understand?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for dwells that I've seen before include "Lodges or resides" , "pauses" , "Establishes oneself" , "Lives in, inhabits" .)