One, possibly nosy, causes displeasure (6)
I believe the answer is:
annoys
'causes displeasure' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot understand how they can define each other.
'one possibly nosy' is the wordplay.
'one' becomes 'an' (an apple is one apple).
'possibly' indicates anagramming the letters.
'nosy' is an anagram of 'noys'.
'an'+'noys'='ANNOYS'
Can you help me to learn more?
(Other definitions for annoys that I've seen before include "Irks, vexes" , "Irritates, vexes" , "'Irritates, irks (6)'" , "Harasses, pesters" , "Narks" .)