Joint in middle of Warsaw causes annoyance (7)
I believe the answer is:
rankles
'causes annoyance' is the definition.
Although both the answer and definition are verbs in their -s form, I don't see how one could define the other.
'joint in middle of warsaw' is the wordplay.
'joint' becomes 'ankle' (joint of the body).
'in' indicates putting letters inside.
'middle of' indicates the central letters.
The centre of 'warsaw' is 'rs'.
'ankle' inserted inside 'rs' is 'RANKLES'.
Can you help me to learn more?
(Other definitions for rankles that I've seen before include "Riles" , "Goes on vexing" , "Continues to hurt" , "Causes lasting and bitter annoyance" .)