Favourite holding love lines in verse (6)
I believe the answer is:
poetry
'verse' is the definition.
('poetry' can be a synonym of 'verse')
'favourite holding love lines' is the wordplay.
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
'holding' means one lot of letters goes inside another.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'lines' becomes 'ry' (abbreviation for railway).
'pet' enclosing 'o' is 'poet'.
'poet'+'ry'='POETRY'
'in' acts as a link.
(Other definitions for poetry that I've seen before include "Metrical writing" , "Verse" , "Literature in metrical form" , "The product of Tennyson or Longfellow" , "Lines" .)