Stop initial offence being capital (5)
I believe the answer is:
quito
'being capital' is the definition.
(capital of Ecuador)
'stop initial offence' is the wordplay.
'stop' becomes 'quit' (synonyms).
'initial' suggests taking the first letters.
The initial letter of 'offence' is 'o'.
'quit'+'o'='QUITO'
(Other definitions for quito that I've seen before include "Ecuadorian capital" , "City" , "S America" , "Capital of Ecuador" .)