Close the main line (4)
I believe the answer is:
seal
'close' is the definition.
(sealing is a kind of closing)
'the main line' is the wordplay.
'the main' becomes 'sea' ('sea' can be a synonym of 'main').
'line' becomes 'l'.
'sea'+'l'='SEAL'
(Other definitions for seal that I've seen before include "Authorise - plug" , "relative of the walrus" , "Secure; aquatic animal" , "Amphibious fish-eating sea mammal" , "Stamped wax; mammal" .)