Bound to make line break important in dictionary (7)
I believe the answer is:
obliged
'bound' is the definition.
('oblige' can be a synonym of 'bind')
'line break important in dictionary' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'break' means one lot of letters goes inside another (inserted letters break into the word).
'important' becomes 'big' (similar in meaning).
'in' means one lot of letters goes inside another.
'dictionary' becomes 'OED' (Oxford English Dictionary).
'l' going inside 'big' is 'blig'.
'blig' placed within 'oed' is 'OBLIGED'.
'to make' acts as a link.
(Other definitions for obliged that I've seen before include "Constrained" , "required" , "Did favours - compelled" , "Bound by a favour rendered" , "Bound by duty or gratitude" .)