Revolutionary queen I love for so long (7)
I believe the answer is:
cheerio
'for so long' is the definition.
(both can mean goodbye)
'revolutionary queen i love' is the wordplay.
'revolutionary' becomes 'che' (Che Guevara the Marxist revolutionary).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'love' becomes 'o' (love means zero in tennis).
'che'+'er'+'i'+'o'='CHEERIO'
(Other definitions for cheerio that I've seen before include "Ta-ta" , "A hearty farewell, begins with applause" , "Informal farewell" , "Informal goodbye, sounds blithe" , "An informal hearty goodbye" .)