King and Queen visiting important Irish county (5)
I believe the answer is:
kerry
'irish county' is the definition.
(I've seen this before)
'king and queen visiting important' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'and' means one lot of letters go next to another (I've seen this in other clues).
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'visiting' indicates putting letters inside.
'important' becomes 'key' ('key' can be similar in meaning to 'important').
'r' after 'r' is 'rr'.
'rr' placed inside 'key' is 'KERRY'.
(Other definitions for kerry that I've seen before include "County of Ireland" , "County of Tralee" , "Irish county" , "County < girl" .)