Encourage one love for so long (7)
I believe the answer is:
cheerio
'so long' is the definition.
('cheerio' can be a synonym of 'so long')
'encourage one love' is the wordplay.
'encourage' becomes 'cheer' (cheering is a kind of encouraging).
'one' becomes 'i' (Roman numeral).
'love' becomes 'o' (In tennis, 'love' means 'zero').
'cheer'+'i'+'o'='CHEERIO'
'for' acts as a link.
(Other definitions for cheerio that I've seen before include "A hearty farewell, begins with applause" , "An informal hearty goodbye" , "Say this before drinking" , "for so long!" , "See you" .)