Agent initially expected to omit a line (8)
I believe the answer is:
emissary
'agent' is the definition.
(an emissary is an agent or representative)
'initially expected to omit a line' is the wordplay.
'initially' suggests taking the first letters.
'to omit' becomes 'miss' ('miss' can be a synonym of 'omit').
'line' becomes 'ry' (abbreviation for railway).
The first letter of 'expected' is 'e'.
'e'+'miss'+'a'+'ry'='EMISSARY'
(Other definitions for emissary that I've seen before include "Person sent on a diplomatic mission" , "Delegate" , "One sent" , "Diplomatic messenger" , "Representative sent on a mission" .)