Irish city of fifty thousand concealing one awful crime (8)
I believe the answer is:
limerick
'irish city' is the definition.
(I know that Limerick is a irish city)
'fifty thousand concealing one awful crime' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'thousand' becomes 'k' (synonyms).
'concealing' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'awful' indicates anagramming the letters.
'l'+'k'='lk'
'crime' with letters rearranged gives 'meric'.
'i'+'meric'='imeric'
'lk' enclosing 'imeric' is 'LIMERICK'.
'of' is the link.
(Other definitions for limerick that I've seen before include "City of humorous verse" , "port used in Ireland" , "Rhyme" , "Irish county and port" , "Humorous short poem" .)