Vexing one, pranksters at centre in capital city (7)
I believe the answer is:
irksome
'vexing' is the definition.
(both can mean annoying)
'one pranksters at centre in capital city' is the wordplay.
'one' becomes 'i' (Roman numeral).
'at centre' means to look at the middle letters.
'in' indicates putting letters inside.
'capital city' becomes 'rome' (capital of Italy).
The central letters of 'pranksters' are 'ks'.
'ks' put into 'rome' is 'rksome'.
'i'+'rksome'='IRKSOME'
(Other definitions for irksome that I've seen before include "Smokier (anag.)" , "Tiresome, irritating" , "Annoying or tiresome" , "bugging" , "Tediously annoying" .)