Monica, to Ross, on "Friends" (3)
I believe the answer is:
sis
'monica to ross on friends' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sis that I've seen before include "Family member (informal)" , "Girl" , "Relative's" , "spies ..." , "Secret Intelligence Service (abbreviation)" .)