A current boast about Polish city (6)
I believe the answer is:
cracow
'polish city' is the definition.
(I've seen this before)
'a current boast about' is the wordplay.
'a current' becomes 'ac' (alternating current).
'boast' becomes 'crow' (to crow about something is to boast about it).
'about' indicates putting letters inside.
'ac' inserted into 'crow' is 'CRACOW'.
(Other definitions for cracow that I've seen before include "Capital at one time" , "Former Polish capital" , "foreign city" , "Polish city" , "E European city" .)