Have one? I'll be blessed if I will! (6)
I believe the answer is:
sneeze
'have' is the definition.
The answer and definition can be both related to bodily activities as well as being verbs in their base 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 sneeze that I've seen before include "Symptom of cold" , "Explosive nasal release" , "reason for blessing" , "Audible and uncontrolled expulsion of air through nose and mouth" , "Sign of the common cold" .)