It's a sign of love, fellows! (4)
I believe the answer is:
omen
'it's a sign' is the definition.
(omen is a kind of sign)
'love fellows' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'fellows' becomes 'men' (fellow can mean a man).
'o'+'men'='OMEN'
'of' is the link.
(Other definitions for omen that I've seen before include "Prophetic significance" , "writing on the wall" , "Auspice" , "Forecast" , "Event regarded as a prophetic sign of good or evil" .)