Dubliner possibly misusing his main river at first (8)
I believe the answer is:
irishman
'dubliner possibly' is the definition.
(dubliner is a kind of Irishman)
'misusing his main river at first' is the wordplay.
'misusing' is an anagram indicator.
'at first' indicates taking the first letters.
The first letter of 'river' is 'r'.
'his'+'main'+'r'='hismainr'
'hismainr' is an anagram of 'IRISHMAN'.
(Other definitions for irishman that I've seen before include "Dublin native" , "Dubliner, say" , "Eg, Dublin male" , "One from Eire" , "National" .)