A second carrier is off course (6)
I believe the answer is:
astray
'off course' is the definition.
(I've seen this before)
'a second carrier' is the wordplay.
'a second' becomes 'as' (I've seen this in other clues).
'carrier' becomes 'tray' (I've seen this before).
'as'+'tray'='ASTRAY'
'is' acts as a link.
(Other definitions for astray that I've seen before include "Wandering lost" , "Off the correct path" , "Off the correct track" , "Away from the right path like a satyr (6)" , "Gone off the right path" .)