Official of county Roscommon, initially (5)
I believe the answer is:
mayor
'official' is the definition.
(I have seen 'Municipal official' mean 'mayor' so perhaps 'official' could also mean 'mayor')
'county roscommon initially' is the wordplay.
'county' becomes 'mayo' (county of Ireland).
'initially' suggests taking the first letters.
The first letter of 'roscommon' is 'r'.
'mayo'+'r'='MAYOR'
'of' acts as a link.
(Other definitions for mayor that I've seen before include "Head of a city council" , "Municipal leader" , "City leader" , "Town's chief citizen." , "Municipal official" .)