Runner is more complacent crossing line (8)
I believe the answer is:
smuggler
'runner' is the definition.
(run can mean to smuggle banned goods)
'more complacent crossing line' is the wordplay.
'more complacent' becomes 'smugger' (smug can mean complacent).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'line' becomes 'l' (used when specifying particular lines from a poem).
'smugger' placed around 'l' is 'SMUGGLER'.
'is' acts as a link.
(Other definitions for smuggler that I've seen before include "One who imports or exports without paying duties" , "He tries to evade customs barriers" , "Exciseman's foe" , "Transporter of illegal goods" , "Runner" .)