Bring love into it, which is tedious (6)
I believe the answer is:
boring
'is tedious' is the definition.
('boring' can be a synonym of 'tedious')
'bring love into it' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'into it' is an insertion indicator.
'bring' enclosing 'o' is 'BORING'.
'which' acts as a link.
(Other definitions for boring that I've seen before include "Making holes" , "Tedious, tiresome" , "Drilling - it's monotonous" , "Tedious, of no interest" , "flat" .)