They're in it for the long haul (6)
'they're in it for the long haul' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for semis that I've seen before include "Suburban houses" , "They transport" , "Paired houses" .)
(Another definition for lifers that I've seen is " They serve very long prison terms".)