Note with a line that's negligible (7)
I believe the answer is:
minimal
'negligible' is the definition.
(similar in meaning)
'note with a line' is the wordplay.
'note' becomes 'minim' (type of musical note).
'with' means one lot of letters go next to another.
'line' becomes 'l' (used when specifying particular lines from a poem).
'minim'+'a'+'l'='MINIMAL'
'that's' acts as a link.
(Other definitions for minimal that I've seen before include "Very little" , "Least possible; negligible" , "Token" , "Of the least possible size" , "Very small, the least possible (7)" .)