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' (I've seen this before).
'initially' suggests taking the first letters.
The initial letter of 'roscommon' is 'r'.
'mayo'+'r'='MAYOR'
'of' is the 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" .)