Irascible Republican lawyer in America (5)
I believe the answer is:
ratty
'irascible' is the definition.
'republican lawyer in america' is the wordplay.
'republican' becomes 'r' (abbreviation).
'lawyer in america' becomes 'atty' (abbreviation for attorney).
'r'+'atty'='RATTY'
(Other definitions for ratty that I've seen before include "Bad tempered" , "Character created by Kenneth Grahame" , "Short" , "Cross - character in "The Wind in the Willows"" , "describing vermin" .)