Law must be broken in cheerful port city (6)
I believe the answer is:
galway
'port city' is the definition.
(Galway is a port city in Ireland)
'law must be broken in cheerful' is the wordplay.
'must be broken' indicates an anagram.
'in' indicates putting letters inside.
'cheerful' becomes 'gay' (synonyms).
'law' anagrammed gives 'lwa'.
'lwa' going into 'gay' is 'GALWAY'.
(Other definitions for galway that I've seen before include "flautist" , "Connacht city" , "Irish city" , "County of W. Ireland" , "Western City of the Tribes" .)