Line used in good books showing taste (7)
I believe the answer is:
flavour
'taste' is the definition.
(I know that taste can be written as flavour)
'line used in good books' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'used in' indicates putting letters inside.
'good books' becomes 'favour' (as in being in someone's good books).
'l' going inside 'favour' is 'FLAVOUR'.
'showing' acts as a link.
(Other definitions for flavour that I've seen before include "Season" , "Distinctive taste" , "property" , "essential quality" , "Characteristic quality of something" .)