Eats in lines, causing unruly behaviour (9)
I believe the answer is:
rowdiness
'unruly behaviour' is the definition.
(I've seen this before)
'eats in lines' is the wordplay.
'eats' becomes 'dines' (dining is a kind of eating).
'in' means one lot of letters goes inside another.
'lines' becomes 'rows' (row is a kind of line).
'dines' placed into 'rows' is 'ROWDINESS'.
'causing' acts as a link.
(Other definitions for rowdiness that I've seen before include "Unruly behaviour" , "Noise, disorder" , "noisy behaviour" , "bad behaviour" , "yobbery" .)