Having learned, repeat city is taking part in ceremony (6)
I believe the answer is:
recite
'having learned repeat' is the definition.
(to recite is to repeat something pre-learned)
'city is taking part in ceremony' is the wordplay.
'city' becomes 'EC' (the City of London postcode).
'is taking part in' indicates putting letters inside.
'ceremony' becomes 'rite' (rite is a kind of ceremony).
'ec' put within 'rite' is 'RECITE'.
(Other definitions for recite that I've seen before include "deliver lines that have been recalled" , "Repeat, recount" , "Repeat aloud from memory as for audience" , "Speak by heart aloud" , "Declaim, recount" .)